Annual report pursuant to Section 13 and 15(d)

Share-based Compensation - Summary of Stock Option Activity (Detail)

v3.10.0.1
Share-based Compensation - Summary of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares        
Beginning Balance, Shares 528,857 474,359 374,685  
Granted, Shares 0 92,879 140,740  
Exercised, Shares (81,100) (38,381) (41,066)  
Forfeited or expired, Shares (19,374) 0 0  
Ending balance, Shares 428,383 528,857 474,359 374,685
Options exercisable, Shares 341,993      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]        
Beginning balance (in dollars per share) $ 20.57 $ 17.87 $ 13.90  
Granted (in dollars per share) 0.00 33.93 26.99  
Exercised (in dollars per share) 17.09 19.51 12.92  
Forfeited or expired (in dollars per share) 31.25 0.00 0.00  
Ending Balance (in dollars per share) 20.74 $ 20.57 $ 17.87 $ 13.90
Options exercisable (in dollars per share) $ 18.11      
Average Remaining Contractual Term and Aggregate Intrinsic Value        
Average remaining contractual term 6 years 5 months 7 years 5 months 8 years 1 month 8 years 6 months
Average remaining contractual term, Options exercisable 6 years 1 month      
Aggregate intrinsic value, Options outstanding $ 2.8 $ 6.7 $ 7.8 $ 2.8
Aggregate intrinsic value, Options exercisable $ 2.8