Annual report pursuant to Section 13 and 15(d)

Accounting Policies - Additional Information (Detail)

v3.8.0.1
Accounting Policies - Additional Information (Detail)
 12 Months Ended
Jan. 01, 2017
USD ($)
Dec. 31, 2017
USD ($)
Financial_Institutions
Office
Dec. 31, 2016
USD ($)
Office
Dec. 31, 2015
USD ($)
Office
Accounting Policies [Line Items]        
Number of financial institutions | Financial_Institutions   4    
Allowance for commissions receivable   $ 0 $ 0  
Advertising costs   $ 824,000 1,200,000 $ 1,100,000
Income tax benefit realized, percentage   50.00%    
Uncertain tax positions   $ 0 0  
Commission's receivable settled period   10 days    
Tax benefit from stock-based award activity included in provision for income taxes   $ 2,900,000    
Operating lease obligations   86,897,000    
Marketable securities, available for sale   $ 125,659,000 $ 104,929,000  
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    
Level 3 [Member]        
Accounting Policies [Line Items]        
Investments   0    
Accounting Standards Update 2016-02 [Member]        
Accounting Policies [Line Items]        
Operating lease obligations   86,900,000    
Accounting Standards Update 2016-09 [Member]        
Accounting Policies [Line Items]        
Tax benefit from stock-based award activity included in provision for income taxes   $ 2,900,000    
Cumulative effect adjustments, net of tax $ 52,000      
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
Maximum [Member]        
Accounting Policies [Line Items]        
Property and equipment, estimated useful lives   7 years    
Minimum [Member]        
Accounting Policies [Line Items]        
Property and equipment, estimated useful lives   3 years    
Average AA Credit Rating [Member] | Accounting Standards Update 2016-13 [Member]        
Accounting Policies [Line Items]        
Marketable securities, available for sale   $ 125,700,000    
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% 1.00%