Fair Value Measurements (Tables)
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6 Months Ended | 
Jun. 30, 2018  | 
| Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis | 
 
 The following tables classify the fair value hierarchy of Fortress's financial instruments, exclusive of National's financial instruments, measured at fair value as of June 30, 2018 and December 31, 2017:
        |       |        |    Fair Value Measurement as of June 30, 2018   |        |     |   ($ in thousands)   |        |    Level 1   |        |    Level 2   |        |    Level 3   |        |    Total   |        |     |   Assets   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Long-term investments, at fair value   |        |    $   |    -   |        |    $   |    -   |        |    $   |    565   |        |    $   |    565   |        |     |   Total   |        |    $   |    -   |        |    $   |    -   |        |    $   |    565   |        |    $   |    565   |        |     |       |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Liabilities   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Warrant liabilities   |        |    $   |    -   |        |    $   |    -   |        |    $   |    -   |        |    $   |    -   |        |     |   Helocyte Convertible Note, at fair value   |        |        |    -   |        |        |    -   |        |        |    333   |        |        |    333   |        |     |   Caelum Convertible Note, at fair value   |        |        |    -   |        |        |    -   |        |        |    10,240   |        |        |    10,240   |        |     |   Total   |        |    $   |    -   |        |    $   |    -   |        |    $   |    10,573   |        |    $   |    10,573   |        |               |       |        |     Fair Value Measurement as of December 31, 2017   |        |     |   ($ in thousands)   |        |    Level 1   |        |    Level 2   |        |    Level 3   |        |    Total   |        |     |   Assets   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Long-term investments, at fair value   |        |    $   |       |        |    $   |       |        |    $   |    1,390   |        |    $   |    1,390   |        |     |   Total   |        |    $   |       |        |    $   |       |        |    $   |    1,390   |        |    $   |    1,390   |        |     |       |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Liabilities   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Warrant liabilities   |        |    $   |       |        |    $   |       |        |    $   |    87   |        |    $   |    87   |        |     |   Caelum Convertible Note, at fair value   |        |        |       |        |        |       |        |        |    10,059   |        |        |    10,059   |        |     |   Helocyte Convertible Note, at fair value   |        |        |       |        |        |       |        |        |    4,700   |        |        |    4,700   |        |     |   Total   |        |    $   |       |        |    $   |       |        |    $   |    14,846   |        |    $   |    14,846   |        |         
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| Schedule of changes in fair value of financial instruments | 
  The table below provides a roll-forward of the changes in fair value of Level 3 financial instruments for the six months ended June 30, 2018:        |   ($ in thousands)   |        |    Investment in  Origo   |        |    Helocyte  Convertible  Note, at fair  value   |        |    Caelum  Convertible  Note, at fair  value   |        |    Warrants  issued and  issuable   |        |    Warrant  liabilities   |        |    Total   |        |     |   Balance at December 31, 2017   |        |    $   |    1,390   |        |    $   |    4,700   |        |    $   |    10,059   |        |    $   |    5,597   |        |    $   |    87   |        |    $   |    21,833   |        |     |   Payment of convertible note   |        |        |    -   |        |        |    (4,076)   |        |        |    -   |        |        |    -   |        |        |    -   |        |        |    (4,076)   |        |     |   Reclassification of warrant liability from debt to equity   |        |        |    -   |        |        |    -   |        |        |    -   |        |        |    (13,615)   |        |        |    -   |        |        |    (13,615)   |        |     |       |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Change in fair value of investments   |        |        |    (825)   |        |        |    -   |        |        |    -   |        |        |    -   |        |        |    -   |        |        |    (825)   |        |     |   Change in fair value of convertible notes   |        |        |    -   |        |        |    (291)   |        |        |    181   |        |        |    -   |        |        |    -   |        |        |    (110)   |        |     |   Change in fair value of derivative liabilities   |        |        |    -   |        |        |    -   |        |        |    -   |        |        |    8,018   |        |        |    (87)   |        |        |    7,931   |        |     |   Balance at June 30, 2018   |        |    $   |    565   |        |    $   |    333   |        |    $   |    10,240   |        |    $   |    0   |        |    $   |    0   |        |    $   |    11,138   |        |          
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| Warrants Issuable [Member] | 
 
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques | 
 Accordingly, at March 15, 2018, the date of the amendment, the fair value of the warrants issued by National (represents   44% of the warrants issued to non-Fortress shareholders) was $ 13.6 million. Such valuation (using level 3 inputs) was determined by use of the Black-Scholes option pricing model using the following assumptions:      |       |        |    March 15, 2018   |        |     |   Dividend yield   |        |        |       |    %   |     |   Expected volatility   |        |        |    59.23   |    %   |     |   Risk-free interest rate   |        |        |    2.42   |    %   |     |   Life (in years)   |        |        |    3.49   |    %   |         
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| Fair Value for Derivative Contingently Issuable Warrant Liabilities | 
  The following table shows the fair value of the warrant liability on the Condensed Consolidated Balance Sheets as of March 31, 2018 and September 30, 2017:        |   ($ in thousands)   |        |    National’s  Warrants   |        |     |   Beginning balance at September 30, 2017   |        |    $   |    5,597   |        |     |   Change in fair value of derivative liability   |        |        |    8,018   |        |     |   Ending balance at March 15, 2018   |        |        |    13,615   |        |     |   Reclassification of warrant to equity   |        |        |    (13,615)   |        |     |   Ending balance at March 31, 2018   |        |    $   |       |        |         
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| Helocyte [Member] | 
 
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| Fair Value for Derivative Contingently Issuable Warrant Liabilities | 
 The fair value of Helocyte’s warrant liability, which was issued in connection with Helocyte’s convertible note (see Note 11), as of June 30, 2018 approximated $ 0, as the probability of the conversion of the underlying notes approximated $ 0. The table below provides a summary:         |    ($ in thousands)   |        |    Fair Value of  Derivative  Warrant  Liability   |        |     |   Beginning balance at January 1, 2018   |        |    $   |    87   |        |     |   Change in fair value of derivative liabilities   |        |        |    (87)   |        |     |   Ending balance at June 30, 2018   |        |    $   |       |        |         
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| Helocyte [Member] | Convertible Debt [Member] | 
 
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| Fair Value for Derivative Contingently Issuable Warrant Liabilities | 
 A summary of the weighted average (in aggregate) significant unobservable inputs (Level 3 inputs) used in measuring Helocyte’s convertible debt that is categorized within Level 3 of the fair value hierarchy as of June 30, 2018, the fair value approximated cost, as the convertible note approaches maturity, is as follows:         |    ($ in thousands)   |        |    Helocyte  Convertible  Note, at Fair  value   |        |     |   Beginning balance at January 1, 2018   |        |    $   |    4,700   |        |     |   Payment of convertible notes   |        |        |    (4,076)   |        |     |   Change in fair value of convertible notes   |        |        |    (291)   |        |     |   Ending balance at June 30, 2018   |        |    $   |    333   |        |         
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| National Warrant Liabilities [Member] | 
 
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques | 
 A summary of the weighted averages (in aggregate) of significant unobservable inputs (Level 3 inputs) used in measuring National’s warrants that are categorized within Level 3 of the fair value hierarchy as of March 31, 2018 is as follows:         |       |        |    March 31, 2018   |        |     |   Risk-free interest rate   |        |    1.63%  2.56%   |        |     |   Expected dividend yield   |        |    %   |        |     |   Expected term in years   |        |    0.18  4.11   |        |     |   Remaining volatility   |        |    56.4%  386.1%   |        |     |   Strike price   |        |    $0.01  $10.00   |        |          
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| Fair Value for Derivative Contingently Issuable Warrant Liabilities | 
    |    ($ in thousands)   |        |    Fair Value of  Derivative  Warrants   |        |     |   Beginning balance at September 30, 2017   |        |    $   |    202   |        |     |   Trading revenue gain   |        |        |    1,349   |        |     |   Ending balance at March 31, 2018   |        |    $   |    1,551   |        |            
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| Caelum [Member] | Convertible Debt [Member] | 
 
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| Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques | 
 Caelum’s convertible debt, which is guaranteed by the Company, is measured at fair value using the Monte Carlo simulation valuation methodology. A summary of the weighted average (in aggregate) significant unobservable inputs (Level 3 inputs) used in measuring Caelum’s convertible debt that is categorized within Level 3 of the fair value hierarchy as of June 30, 2018 is as follows:        |       |        |    June 30, 2018   |        |     |   Risk-free interest rate   |        |        |    1.89%- 2.37   |    %   |     |   Expected dividend yield   |        |        |    -   |    %   |     |   Expected term in years   |        |        |    0.21  1.21   |        |     |   Expected volatility   |        |        |    53.0   |    %   |         
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| Fair Value for Derivative Contingently Issuable Warrant Liabilities | 
    |    ($ in thousands)   |        |    Caelum  Convertible  Note, at fair  value   |        |     |   Beginning balance at January 1, 2018   |        |    $   |    10,059   |        |     |   Change in fair value of convertible notes   |        |        |    181   |        |     |   Ending balance at June 30, 2018   |        |    $   |    10,240   |        |         
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| National Holdings Corporation [Member] | 
 
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| Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis | 
 
   The following table shows the fair values hierarchy of National's financial instruments measured at fair value on a recurring basis on the Condensed Consolidated Balance Sheets as of March 31, 2018 and September 30, 2017:         |       |        |     Fair Value Measurement as of March 31, 2018   |        |     |       |        |    Level 1   |        |    Level 2   |        |    Level 3   |        |    Total   |        |     |   Assets   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   National   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Securities owned, at fair value   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Corporate stocks   |        |    $   |    32   |        |    $   |       |        |    $   |       |        |    $   |    32   |        |     |   Municipal bonds   |        |        |       |        |        |    1,183   |        |        |       |        |        |    1,183   |        |     |   Restricted stock   |        |        |       |        |        |    1,046   |        |        |       |        |        |    1,046   |        |     |   Warrants   |        |        |       |        |        |    -   |        |        |    1,551   |        |        |    1,551   |        |     |   Total   |        |    $   |    32   |        |    $   |    2,229   |        |    $   |    1,551   |        |    $   |    3,812   |        |     |       |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Liabilities   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   National   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Securities sold, but not yet purchased at fair value:   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Corporate stocks   |        |        |    2   |        |        |       |        |        |       |        |        |    2   |        |     |   Corporate debt   |        |        |       |        |        |    6   |        |        |       |        |        |    6   |        |     |   Contingent consideration   |        |        |       |        |        |       |        |        |    889   |        |        |    889   |        |     |   Total   |        |    $   |    2   |        |    $   |    6   |        |    $   |    889   |        |    $   |    897   |        |            
  
     |       |        |     Fair Value Measurement as of September 30, 2017   |        |     |       |        |    Level 1   |        |    Level 2   |        |    Level 3   |        |    Total   |        |     |   Assets   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   National   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Securities owned, at fair value   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Corporate stocks   |        |    $   |    116   |        |    $   |       |        |    $   |       |        |    $   |    116   |        |     |   Municipal bonds   |        |        |       |        |        |    1,239   |        |        |       |        |        |    1,239   |        |     |   Restricted stock   |        |        |       |        |        |    82   |        |        |       |        |        |    82   |        |     |   Warrants   |        |        |       |        |        |       |        |        |    548   |        |        |    548   |        |     |   Total   |        |    $   |    116   |        |    $   |    1,321   |        |    $   |    548   |        |    $   |    1,985   |        |     |       |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Liabilities   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   National   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Securities sold, but not yet purchased at fair value   |        |        |        |        |        |        |        |        |        |        |        |        |        |     |   Municipal bonds   |        |        |       |        |        |    151   |        |        |       |        |        |    151   |        |     |   Contingent consideration   |        |        |       |        |        |       |        |        |    311   |        |        |    311   |        |     |   Warrants issued - National   |        |        |       |        |        |       |        |        |    5,597   |        |        |    5,597   |        |     |   Total   |        |    $   |       |        |    $   |    151   |        |    $   |    5,908   |        |    $   |    6,059   |        |            
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