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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, 2018
$ / shares
shares
Shares  
Nonvested, beginning of period, in shares | shares 637,667
Granted, in shares | shares 185,292
Vested, in shares | shares (206,997)
Forfeited, in shares | shares (21,028)
Nonvested, end of period, in shares | shares 594,934
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period, in usd per share | $ / shares $ 135.86
Granted, in usd per share | $ / shares 301.56
Vested, in usd per share | $ / shares 130.29
Forfeited, in usd per share | $ / shares 148.43
Nonvested, end of period, in usd per share | $ / shares $ 188.96