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Share-Based Compensation - Summary of Performance-Based Option Activity (Detail) - Performance Shares [Member] - Share Based Compensation Plans [Member] - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Shares    
Options Outstanding at Beginning of Period 366,377  
Options, Granted 0  
Options, Exercised (43,849)  
Options, Forfeited or expired 0  
Options Outstanding at End of Period 322,528 366,377
Options Exercisable at End of Period 322,528  
Weighted Average Exercise Price (per share)    
Weighted Average Exercise Price at Beginning of Period $ 13.50  
Weighted Average Exercise Price, Granted 0  
Weighted Average Exercise Price, Exercised (13.64)  
Weighted Average Exercise Price, Forfeited or expired 0  
Weighted Average Exercise Price at End of Period 13.48 $ 13.50
Weighted Average Exercise Price Exercisable at End of Period $ 13.48  
Weighted Average Remaining Contractual Term (in years)    
Weighted Average Remaining Contractual Term (in years) 2 years 10 months 24 days 3 years 1 month 6 days
Weighted Average Remaining Contractual Term Exercisable (in years) 2 years 10 months 24 days  
Aggregate Intrinsic Value    
Aggregate Intrinsic Value, Options Outstanding $ 12.2 $ 7.7
Aggregate Intrinsic Value, Options Exercisable $ 12.2