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Summary of All Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Aggregate Intrinsic Value, Options Exercisable at December 31, 2017 $ 4.4 $ 1.3 $ 0.6
Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Shares/Options Outstanding at December 31, 2016 1,273,754    
Number of Options, Exercised (511,453)    
Number of Options, Forfeited (8,753)    
Number of Options, Expired (510)    
Number of Shares/Options Outstanding at December 31, 2017 753,038 1,273,754  
Number of Options Exercisable at December 31, 2017 704,858    
Weighted Average Grant Date Fair Value (Exercise Price), Outstanding at December 31, 2016 $ 11.13    
Weighted Average Exercise Price, Exercised 11.32    
Weighted Average Exercise Price, Forfeited 14.16    
Weighted Average Exercise Price, Expired 13.32    
Weighted Average Grant Date Fair Value (Exercise Price), Outstanding at December 31, 2017 10.96 $ 11.13  
Weighted Average Exercise Price, Options Exercisable at December 31, 2017 $ 10.74    
Weighted Average Remaining Contractual Life, Options Outstanding 4 years 4 months 24 days 5 years 1 month 6 days  
Weighted Average Remaining Contractual Life, Options Exercisable at December 31, 2017 4 years 2 months 12 days    
Aggregate Intrinsic Value, Options Exercised $ 4.4    
Aggregate Intrinsic Value, Options Outstanding 8.1 $ 6.0  
Aggregate Intrinsic Value, Options Exercisable at December 31, 2017 $ 7.7