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Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) Office | Sep. 30, 2018 Office | Sep. 30, 2019 USD ($) Office | Sep. 30, 2018 Office | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($)
Accounting Policies [Line Items] |  |  |  |  |  |
Commission's receivable settled period |  |  | 10 days |  |  |
Number of offices | Office |  |  | 82 |  |  |
Operating lease right-of-use assets | $ 90,165,000 |  | $ 90,165,000 |  |  | $ 0
Operating lease liabilities | 81,816,000 |  | 81,816,000 |  |  |
Prepaid Rent Reclassified to Other Assets, Current | $ 12,352,000 |  | $ 12,352,000 |  |  | $ 6,368,000
Customer Concentration Risk [Member] | Total revenues [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Concentration risk percentage |  |  | 10.00% | 10.00% |  |
Customer Concentration Risk [Member] | Commissions receivable [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Concentration risk percentage |  |  | 10.00% | 10.00% |  |
Geographic Concentration Risk [Member] | Total revenues [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Concentration risk percentage | 10.00% | 10.00% | 10.00% | 10.00% |  |
Number of offices | Office | 0 | 0 | 0 | 0 |  |
Accounting Standards Update 2016-02 [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Operating lease right-of-use assets |  |  |  |  | $ 76,700,000 |
Operating lease liabilities |  |  |  |  | 76,700,000 |
Prepaid Rent Reclassified to Other Assets, Current |  |  |  |  | 462,000 |
Accounting Standards Update 2016-02 [Member] | Deferred rent reclassified to Operating Lease ROU Asset [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Operating lease right-of-use assets |  |  |  |  | 5,600,000 |
Accounting Standards Update 2016-02 [Member] | Prepaid rent reclassified to Operating Lease ROU Asset [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Operating lease right-of-use assets |  |  |  |  | $ 13,400,000 |
Accounting Standards Update 2018-13 [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Contingent consideration liability | $ 2,900,000 |  | $ 2,900,000 |  |  |
Accounting Standards Update 2018-13 [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Mortgage servicing rights | $ 2,000,000.0 |  | $ 2,000,000.0 |  |  |
Maximum [Member] | Geographic Concentration Risk [Member] | Total revenues [Member] | International Revenues [Member] |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |
Concentration risk percentage | 1.00% | 1.00% | 1.00% | 1.00% |  |

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