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Share-based Compensation - Schedule of Changes in Restricted Stock and Stock Units (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2015
Time-based Awards and Units [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Non-vested at the beginning of the period (in shares) 358,919  
Granted (in shares) 143,469  
Vested (in shares) (157,807)  
Cancelled (in shares) (18,707)  
Non-vested at the end of the period (in shares) 325,874  
Non-vested at the beginning of period (in dollars per share) $ 38.27  
Granted (in dollars per share) 46.42  
Vested (in dollars per share) 37.04  
Cancelled (in dollars per share) 38.59  
Non-vested at the end of period (in dollars per share) $ 42.44  
Performance-based Awards [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Non-vested at the beginning of the period (in shares) 192,310  
Granted (in shares)   55,330
Vested (in shares) (136,980)  
Non-vested at the end of the period (in shares) 55,330  
Non-vested at the beginning of period (in dollars per share) $ 34.45  
Vested (in dollars per share) 34.99  
Non-vested at the end of period (in dollars per share) $ 33.12  
Performance-based or Market-based Stock Units [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Non-vested at the beginning of the period (in shares) 151,575  
Granted (in shares) 94,902  
Cancelled (in shares) (4,613)  
Non-vested at the end of the period (in shares) 241,864  
Non-vested at the beginning of period (in dollars per share) $ 43.54  
Granted (in dollars per share) 54.49  
Cancelled (in dollars per share) 49.09  
Non-vested at the end of period (in dollars per share) $ 47.73