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Stock-based Compensation Plans - Restricted Stock and Restricted Stock Units Outstanding (Detail) - $ / shares
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Equity Awards    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Outstanding, Beginning Balance 765,971  
Granted 874,826 940,000
Vested (384,899)  
Forfeited (71,169)  
Outstanding, Ending Balance 1,184,729  
Outstanding, Beginning Balance, weighted average grant date fair value $ 33.62  
Granted, weighted average grant date fair value 32.93 $ 28.18
Vested, weighted average grant date fair value 35.31  
Forfeited, weighted average grant date fair value 33.28  
Outstanding, Ending Balance, weighted average grant date fair value $ 32.58  
Liability Awards    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Outstanding, Beginning Balance 425,018  
Granted 539,028  
Vested (250,796)  
Forfeited (4,342)  
Outstanding, Ending Balance 708,908  
Outstanding, Beginning Balance, weighted average grant date fair value $ 43.48  
Granted, weighted average grant date fair value 25.99  
Vested, weighted average grant date fair value 37.52  
Forfeited, weighted average grant date fair value 31.10  
Outstanding, Ending Balance, weighted average grant date fair value $ 32.36