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Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail) | 12 Months Ended |
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Dec. 31, 2019 USD ($) Office Segment | Dec. 31, 2018 USD ($) Office | Dec. 31, 2017 USD ($) Office | Jan. 01, 2019 USD ($)
Accounting Policies [Line Items] |  |  |  |
Commission's receivable settled period | 10 days |  |  |
Number of offices | Office | 82 |  |  |
Operating lease right-of-use assets | $ 90,535,000 | $ 0 |  |
Operating lease liabilities | 80,917,000 |  |  |
Prepaid Rent Reclassified to Other Assets, Current | 6,067,000 | 6,368,000 |  |
Advertising Costs | $ 889,000 | 1,100,000 | $ 824,000 |
Income tax benefit realized, percentage | 50.00% |  |  |
Property and equipment, amortization method | straight-line |  |  |
Allowance for commissions receivable | $ 0 | $ 0 |  |
Number of reporting units | Segment | 1 |  |  |
Finite-lived intangible asset, amortization method | straight-line |  |  |
Employee Stock Purchase Plan [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Length of purchase intervals | 6 months |  |  |
Expected dividend yield | 0.00% |  |  |
Forfeiture rate | 0.00% |  |  |
Payment of dividend | $ 0 |  |  |
Customer Concentration Risk [Member] | Total revenues [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage | 10.00% | 10.00% | 10.00% |
Customer Concentration Risk [Member] | Commissions receivable [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage | 10.00% | 10.00% | 10.00% |
Geographic Concentration Risk [Member] | Total revenues [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Concentration risk percentage | 10.00% | 10.00% | 10.00% |
Number of offices | Office | 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 |  |  |  | 462,000,000
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
Maximum [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Property and equipment, estimated useful lives | 7 years |  |  |
Finite-lived intangible asset, useful life | 6 years |  |  |
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% |
Minimum [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Property and equipment, estimated useful lives | 3 years |  |  |
Finite-lived intangible asset, useful life | 1 year |  |  |
Capitalization of Internal Labor [Member] |  |  |  |
Accounting Policies [Line Items] |  |  |  |
Property and equipment, estimated useful lives | 5 years |  |  |

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