Quarterly report pursuant to Section 13 or 15(d)

Stock Compensation - Additional Information (Detail)

v3.10.0.1
Stock Compensation - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Apr. 17, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense $ 7.6 $ 7.5 $ 23.6 $ 21.7  
Unrecognized compensation cost related to nonvested common shares $ 82.4   $ 82.4    
Weighted average period for recognition of cost     2 years 9 months 18 days    
Class A Common Stock [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock redeemed, shares 7,899   308,155    
Stock redeemed, value $ 0.2   $ 11.4    
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%   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