Quarterly report pursuant to Section 13 or 15(d)

Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)

v3.8.0.1
Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)
3 Months Ended
Mar. 31, 2018
USD ($)
Office
Mar. 31, 2017
Office
Dec. 31, 2017
USD ($)
Accounting Policies [Line Items]      
Commission's receivable settled period 10 days    
Marketable securities, available for sale $ 130,399,000   $ 125,659,000
Accounting Standards Update 2018-02 [Member]      
Accounting Policies [Line Items]      
Reclassification of stranded tax effects from accumulated other comprehensive income to retained earnings 13,000    
Accounting Standards Update 2016-02 [Member]      
Accounting Policies [Line Items]      
Operating lease obligations $ 83,600,000    
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%  
Number of offices | Office 0 0  
Capitalization of Internal Labor [Member] | Maximum [Member]      
Accounting Policies [Line Items]      
Property and equipment, estimated useful life 7 years    
Capitalization of Internal Labor [Member] | Minimum [Member]      
Accounting Policies [Line Items]      
Property and equipment, estimated useful life 3 years    
Average AA Credit Rating [Member] | Accounting Standards Update 2016-13 [Member]      
Accounting Policies [Line Items]      
Marketable securities, available for sale $ 130,400,000    
Impairment write-downs marketable securities, available for sale $ 0    
International Revenues [Member] | Maximum [Member] | Geographic Concentration Risk [Member] | Total revenues [Member]      
Accounting Policies [Line Items]      
Concentration risk percentage 1.00% 1.00%