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Share-based 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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expenses $ 2.4 $ 2.4 $ 9.7 $ 6.6
Restricted Stock Units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Maximum unrecognized cost for stocks and RSU's $ 19.9   $ 19.9  
Unrecognized compensation cost, weighted average period of recognition     1 year 11 months 5 days  
Time Based Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     3 years  
Awards with Performance Conditions | 2015 LTIP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     1 year  
Awards with Performance Conditions | 2016 LTIP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period     2 years  
Awards with Performance Conditions | 2017 LTIP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     3 years  
Requisite service period     2 years  
Performance adjustment percent     25.00%  
Performance measurement period     1 year