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Share-Based Compensation - Information for Restricted Stock Units under Equity Incentive Plan (Detail) - Restricted stock units
6 Months Ended
Jun. 30, 2017
$ / shares
shares
Shares  
Nonvested, beginning of period, in shares | shares 670,969
Granted, in shares | shares 230,822
Vested, in shares | shares (218,336)
Forfeited, in shares | shares (16,078)
Nonvested, end of period, in shares | shares 667,377
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period, in usd per share | $ / shares $ 106.64
Granted, in usd per share | $ / shares 179.94
Vested, in usd per share | $ / shares 102.07
Forfeited, in usd per share | $ / shares 117.29
Nonvested, end of period, in usd per share | $ / shares $ 133.23