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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Apr. 17, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense $ 7.5 $ 6.2 $ 21.7 $ 17.1  
Unrecognized compensation cost related to nonvested common shares 43.6   $ 43.6    
Weighted average period for recognition of cost     2 years 1 month 6 days    
ASU 2016-09 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Tax (expense) benefit recorded $ 0.2   $ 1.1    
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