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Summary of RSU and PSU Activity (Detail) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Performance Share Unit (PSUs)    
Outstanding Number of Shares    
Outstanding at beginning of period 901,773  
Granted 247,992  
Shares vested (311,492)  
Forfeited (7,750)  
Outstanding at ending of period 830,523 901,773
Weighted Average Fair Value    
Outstanding at beginning of period $ 48.33  
Granted 71.58  
Shares vested 33.09  
Forfeited 56.58  
Outstanding at end of period $ 60.73 $ 48.33
Weighted Average Remaining Contractual Life    
Outstanding at end of period 1 year 4 months 10 days 1 year 2 months 19 days
Restricted Stock Units (RSUs)    
Outstanding Number of Shares    
Outstanding at beginning of period 1,067,613  
Granted 373,630  
Shares vested (292,229)  
Forfeited (108,504)  
Outstanding at ending of period 1,040,510 1,067,613
Weighted Average Fair Value    
Outstanding at beginning of period $ 48.30  
Granted 66.93  
Shares vested 35.92  
Forfeited 51.96  
Outstanding at end of period $ 58.09 $ 48.30
Weighted Average Remaining Contractual Life    
Outstanding at end of period 1 year 7 months 13 days 1 year 5 months 23 days