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Share-Based Compensation - Information for Restricted Stock Units under Equity Incentive Plan (Detail) - Restricted stock units
9 Months Ended
Sep. 30, 2017
$ / shares
shares
Shares  
Nonvested, beginning of period, in shares | shares 670,969
Granted, in shares | shares 239,847
Vested, in shares | shares (223,561)
Forfeited, in shares | shares (38,871)
Nonvested, end of period, in shares | shares 648,384
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period, in usd per share | $ / shares $ 106.64
Granted, in usd per share | $ / shares 180.05
Vested, in usd per share | $ / shares 102.00
Forfeited, in usd per share | $ / shares 122.77
Nonvested, end of period, in usd per share | $ / shares $ 134.43