Annual report pursuant to Section 13 and 15(d)

Stock Compensation - Additional Information (Detail)

v3.8.0.1
Stock Compensation - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Apr. 17, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Recognized compensation expense $ 28,900 $ 23,100 $ 18,300  
Income tax benefits recorded in additional paid-in capital   4,682 $ 8,009  
Unrecognized compensation cost related to nonvested common shares $ 39,600      
Weighted average period for recognition of cost 2 years 1 month 6 days      
Total fair value of shares vested $ 24,000 $ 13,700    
Proceeds from issuance of common stock under the employee stock purchase plan, Shares 240,000 220,844 223,892  
Proceeds from issuance of common stock under the employee stock purchase plan $ 5,479 $ 5,120 $ 4,318  
ASU 2016-09 [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Excess tax benefits recorded in earnings $ 2,600      
Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period of shares from the date of grant 1 year      
Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period of shares from the date of grant 4 years      
Class A Common Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Proceeds from issuance of common stock under the employee stock purchase plan, Shares 240,000 221,000 224,000  
2017 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares that remain available for grant 9,888,500      
2017 Plan [Member] | Class A Common Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares reserved for issuance       10,000,000
2018 ESPP [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Maximum percentage of employee's compensation to purchase common stock 15.00%      
Percentage of price of common stock purchased 85.00%      
2018 ESPP [Member] | Class A Common Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares available for sale under employee stock purchase plan       5,000,000