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Stockholders' Equity - Summary of Activity for Non-vested Three-year PSU awards (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Vested (in shares) (607,231) (179,826) (172,071)
Forfeited or expired (in shares) (184,235) (89,699) (185,056)
PSU Awards      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Non-vested at beginng of period (in shares) 459,014    
Shares issued 300,002    
Vested (in shares) (500,455)    
Forfeited or expired (in shares) (139,605)    
Non-vested at end of period (in shares) 118,956 459,014  
Weighted-Average per Share Fair Value on Grant Date      
Non-vested, Weighted- Average per Share Fair value on Grant Date, beginning of period (in usd per share) $ 49.55    
Granted (in usd per share) 44.88    
Forfeited or expired (in usd per share) 46.43    
Vested (in usd per share) 47.89    
Non-vested, Weighted- Average per Share Fair Value on Grant Date, end of period (in usd per share) $ 48.40 $ 49.55