Quarterly report pursuant to Section 13 or 15(d)

Stock Compensation - Additional Information (Detail)

v3.8.0.1
Stock Compensation - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Apr. 17, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 8.7 $ 6.6  
Unrecognized compensation cost related to nonvested common shares $ 92.4    
Weighted average period for recognition of cost 3 years    
Class A Common Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock redeemed, shares 212,930    
Stock redeemed, value $ 8.7    
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