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Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail) |  | 3 Months Ended |
--- | --- | --- | ---
Jan. 01, 2020 USD ($) | Mar. 31, 2020 USD ($) Office | Mar. 31, 2019 Office | Dec. 31, 2019 USD ($)
Accounting Policies [Line Items] |  |  |  |
Commission's receivable settled period |  | 10 days |  |
Number of offices | Office |  | 82 |  |
Allowance for commissions receivable | $ |  | $ 0 |  | $ 0
Transaction Risk [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage |  | 0.00% | 0.00% |
Transaction Risk [Member] | Total revenues [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage |  | 10.00% | 10.00% |
Transaction Risk [Member] | Commissions receivable, net [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage |  | 10.00% |  |
Geographic Concentration Risk [Member] | Total revenues [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage |  | 10.00% | 10.00% |
Number of offices | Office |  | 1 | 0 |
Accounting Standards Update 2016-13 [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Cumulative effect adjustment, net | $ | $ 33,000,000 |  |  |
Minimum [Member] | Loan [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Loans to Investment Sales and Financing Professionals, Repayment Term |  | 1 year |  |
Minimum [Member] | Advances To Investment Sales And Financing Professionals [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Advances to Investment Sales and Financing Professionals, Repayment Term |  | 5 years |  |
Maximum [Member] | Loan [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Loans to Investment Sales and Financing Professionals, Repayment Term |  | 7 years |  |
Maximum [Member] | Advances To Investment Sales And Financing Professionals [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Advances to Investment Sales and Financing Professionals, Repayment Term |  | 10 years |  |
Maximum [Member] | Geographic Concentration Risk [Member] | Total revenues [Member] | International Revenues [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage |  | 2.00% | 1.00% |

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