Quarterly report pursuant to Section 13 or 15(d)

Stock Compensation - Additional Information (Detail)

v3.21.2
Stock Compensation - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Recognized stock compensation expense $ 14.6 $ 12.8 $ 26.7 $ 25.3
Unrecognized compensation cost related to nonvested common shares $ 93.2   $ 93.2  
Weighted average period for recognition of cost     2 years 1 month 9 days  
Performance-Based Awards [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Performance period     3 years  
Performance-Based Awards [Member] | Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Payout percentage of performance awards     0.00%  
Performance-Based Awards [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Payout percentage of performance awards     200.00%  
2017 Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of shares that remain available for grant 4,832,416   4,832,416  
2018 ESPP [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of price of common stock purchased     85.00%  
Maximum percentage of employee's compensation to purchase common stock 15.00%   15.00%  
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   5,000,000