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Stock-based Compensation Plans - Restricted Stock and Restricted Stock Units Outstanding (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Equity Awards      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Outstanding, Beginning Balance 765,971 436,764 360,415
Granted 888,326 973,491 587,711
Vested (698,563) (525,617) (480,253)
Forfeited (122,676) (118,667) (31,109)
Outstanding, Ending Balance 833,058 765,971 436,764
Outstanding, Beginning Balance, weighted average grant date fair value $ 33.62 $ 59.74 $ 79.60
Granted, weighted average grant date fair value 32.61 28.51 52.29
Vested, weighted average grant date fair value 34.82 43.83 65.21
Forfeited, weighted average grant date fair value 32.91 42.60 64.73
Outstanding, Ending Balance, weighted average grant date fair value $ 31.64 $ 33.62 $ 59.74
Restricted Stock Liability Awards      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Outstanding, Beginning Balance 425,018 308,737 304,504
Granted 543,438 540,128 343,397
Vested (908,912) (374,328) (330,870)
Forfeited (4,342) (49,519) (8,294)
Outstanding, Ending Balance 55,202 425,018 308,737
Outstanding, Beginning Balance, weighted average grant date fair value $ 43.48 $ 65.80 $ 80.33
Granted, weighted average grant date fair value 25.91 35.92 55.92
Vested, weighted average grant date fair value 33.71 51.40 68.71
Forfeited, weighted average grant date fair value 31.10 40.33 74.22
Outstanding, Ending Balance, weighted average grant date fair value $ 32.26 $ 43.48 $ 65.80