Annual report pursuant to Section 13 and 15(d)

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

v3.10.0.1
Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Financial_Institutions
Office
Segment
Dec. 31, 2017
USD ($)
Office
Dec. 31, 2016
USD ($)
Office
Jan. 01, 2019
USD ($)
Accounting Policies [Line Items]        
Number of financial institutions | Financial_Institutions 5      
Custodial escrow accounts $ 2,100,000 $ 0    
Property and equipment, amortization method Straight-line      
Allowance for commissions receivable $ 0 0    
Advertising costs $ 1,100,000 824,000 $ 1,200,000  
Income tax benefit realized, percentage 50.00%      
Commission's receivable settled period 10 days      
Number of offices | Office 80      
Number of reporting units | Segment 1      
Finite-lived intangible asset, amortization method Straight-line      
Marketable securities, available for sale $ 220,645,000 $ 125,659,000    
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 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] | Subsequent Event [Member]        
Accounting Policies [Line Items]        
Operating lease, right of use asset       $ 84,500,000
Operating lease, liability       $ 76,700,000
Accounting Standards Update 2018-13 [Member]        
Accounting Policies [Line Items]        
Contingent consideration liability 2,900,000      
Average AA+ Credit Rating [Member] | Accounting Standards Update 2016-13 [Member]        
Accounting Policies [Line Items]        
Marketable securities, available for sale $ 220,600,000      
Impairment write-downs marketable securities, available for sale   $ 0    
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