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Share-Based Compensation Information for Restricted Stock Units under Equity Incentive Plan (Detail) - Restricted stock units
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Shares  
Nonvested, beginning of period (in shares) | shares 670,969
Granted (in shares) | shares 247,591
Vested (in shares) | shares (228,198)
Forfeited (in shares) | shares (52,695)
Nonvested, end of period (in shares) | shares 637,667
Weighted Average Grant Date Fair Value  
Nonvested, beginning of period (usd per share) | $ / shares $ 106.64
Granted (usd per share) | $ / shares 181.23
Vested (usd per share) | $ / shares 102.47
Forfeited (usd per share) | $ / shares 121.52
Nonvested, end of period (usd per share) | $ / shares $ 135.86