Fortress Biotech, Inc. Quarterly report pursuant to Section 13 or 15(d)

Quarterly report pursuant to Section 13 or 15(d)

Related Party Transactions - Additional Information (Detail)

Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended					3 Months Ended		9 Months Ended
	Jul. 31, 2017	Jun. 30, 2017	Jun. 26, 2017	Mar. 17, 2017	May 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017	Sep. 09, 2016	Mar. 17, 2014
Related Party Transaction [Line Items]
Lease Sharing Agreements Contributions To Property Under Lease								$ 300,000
Equity Method Investment, Ownership Percentage												35.00%
Class of Warrant or Right, Exercise Price of Warrants or Rights			$ 4.02
Proceeds From Convertible Debt								9,914,000	$ 3,018,000
Payments of Financing Costs								$ 1,081,000	0
Avenue Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Payments for Underwriter Fees			$ 2,300,000
Maximum [Member]
Related Party Transaction [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights						$ 4.75		$ 4.75
Minimum [Member]
Related Party Transaction [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights						$ 3.61		$ 3.61
National Holdings Corporation [Member]
Related Party Transaction [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired				56.60%		56.60%		56.60%			56.60%
NSC Note [Member]
Related Party Transaction [Line Items]
Payments for Rent						$ 900,000		$ 2,800,000
NSC 2017 Note [Member]
Related Party Transaction [Line Items]
Debt Instrument, Face Amount										$ 3,250,000
Percentage Of Placement Agent Fee				10.00%				10.00%
Warrant On Sale Price Of Note Percentage				10.00%		10.00%		10.00%
Class of Warrant or Right, Number of Securities Called by Warrants or Rights						716,180		716,180
Class of Warrant or Right, Exercise Price of Warrants or Rights										$ 3.70
NSC 2017 Note [Member] \| Maximum [Member]
Related Party Transaction [Line Items]
Debt Instrument, Face Amount										$ 40,000,000
Caelum Convertible Notes [Member]
Related Party Transaction [Line Items]
Debt Instrument, Face Amount	$ 10,000,000
Proceeds From Convertible Debt	9,900,000					$ 9,900,000
Payments of Financing Costs	$ 900,000					$ 900,000
Debt Instrument Issuance, Percentage of Commission	10.00%
Class of Warrant or Right, Percentage Securities Called by Warrants or Rights	10.00%					10.00%
Class of Warrant or Right, Exercise Price percentage	110.00%					110.00%
Class of Warrant or Right, Exercise Price Description								the exercise price will be $75 million dollars divided by the total number of fully-diluted shares of Common Stock outstanding immediately prior to exercise of the warrant, giving effect to the assumed conversion of all options, warrants, and convertible securities of the Company
Opus Credit Facility [Member]
Related Party Transaction [Line Items]
Payments of Loan Costs						$ 300,000	$ 800,000	$ 0	0
Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party					$ 10,000
Board of Directors Chairman [Member] \| National Holdings Corporation [Member]
Related Party Transaction [Line Items]
Equity Method Investment, Ownership Percentage						4.60%		4.60%
TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Payments for Rent								$ 600,000
Due from Related Parties, Current						$ 100,000		100,000
Checkpoint [Member] \| Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party						$ 15,000		52,000
Founders Agreement [Member] \| Avenue Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Other Fee,Percentage		2.50%	2.50%
Founders Agreement [Member] \| Caelum Convertible Notes [Member]
Related Party Transaction [Line Items]
Other Fee,Percentage						2.50%
Stock Issued During Period, Shares, Conversion of Convertible Securities						218,000
Stock Issued During Period, Value, Conversion of Convertible Securities						$ 200,000
OPPM [Member]
Related Party Transaction [Line Items]
Payments for Rent								100,000
Due from Related Parties, Current						28,000		28,000
Mustang [Member] \| Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party					7,500
Avenue [Member] \| Chord Advisors, LLC [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party					$ 5,000	17,000		58,000
Avenue [Member] \| Founders Agreement [Member]
Related Party Transaction [Line Items]
Stock Issued During Period, Shares, New Issues			158,000
Desk Share Agreements [Member]
Related Party Transaction [Line Items]
Payments for Rent								1,800,000
Desk Share Agreements [Member] \| TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Due from Related Parties, Current						56,000		56,000
Desk Share Agreements [Member] \| OPPM [Member]
Related Party Transaction [Line Items]
Due from Related Parties, Current						100,000		100,000
Shared Services Agreement [Member] \| TG Therapeutics, Inc [Member]
Related Party Transaction [Line Items]
Proceeds from Related Party Agreement						200,000	$ 300,000	800,000	$ 600,000
Due from Related Parties, Current						$ 100,000		$ 100,000