Share-Based Compensation Stock Option Information Related to Equity Incentive Plan (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Options
Outstanding, beginning of period (in shares) | shares	1,010,557.000
Granted (in shares) | shares	116,995
Exercised (in shares) | shares	(302,744)
Forfeited (in shares) | shares	(16,759)
Outstanding, end of period (in shares) | shares	808,049
Vested and expected to vest (in shares) | shares	787,182
Exercisable (in shares) | shares	464,685
Weighted average exercise price
Outstanding, beginning of period (usd per share) | $ / shares	$ 87.24
Granted (usd per share) | $ / shares	179.39
Exercised (usd per share) | $ / shares	71.65
Forfeited (usd per share) | $ / shares	122.97
Outstanding, end of period (usd per share) | $ / shares	105.68
Vested and expected to vest (usd per share) | $ / shares	104.69
Exercisable (usd per share) | $ / shares	$ 84.86
Weighted Average Remaining Contractual Life in Years
Outstanding (in years)	3 years 8 months 9 days
Vested and expected to vest (in years)	3 years 7 months 19 days
Exercisable (in years)	2 years 6 months 25 days
Aggregate Intrinsic Value of In-The-Money Options
Outstanding | $	$ 103,506,420
Vested and expected to vest | $	101,607,636
Exercisable | $	$ 69,197,233