Revenues from Contracts and Significant Customers - presents National’s revenues (Detail) - USD ($) $ in Thousands  | 
3 Months Ended | 6 Months Ended | 
|---|---|---|
Jun. 30, 2018  | 
Jun. 30, 2018  | 
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| Commissions | $ 31,407 | $ 57,025 | 
| Investment banking: | ||
| Underwriting | 6,893 | 9,593 | 
| Private placement | 3,484 | 13,299 | 
| Advisory | 833 | 2,147 | 
| Other | (4) | 156 | 
| Sub-total National revenue from contracts with customers | 42,613 | 82,220 | 
| Other National revenue | 14,406 | 24,321 | 
| Total National revenue | $ 57,019 | $ 106,541 | 
| X | ||||||||||
- Definition The amount of revenue from private placement. No definition available. 
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- Definition The amount of revenue from contracts with customers excluding other revenue. No definition available. 
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- Definition The amount of revenue from contracts with customers including other revenue. No definition available. 
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- Definition Amount of commission revenue from buying and selling securities on behalf of customers. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of fees earned from providing investment advice and research to customers. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- References No definition available. 
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- Definition Amount, after sales adjustments, returns, allowances, and discounts, of revenue classified as other. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Amount of fee and financial service revenue classified as other. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition The difference between the price paid by the public and the contract price less the related expenses. A broker-dealer may underwrite a security offering by contracting to buy the issue either at a fixed price or a price based on selling the offering on a best-effort basis. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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