Stock-Based Compensation - Schedule of Additional Information on Unvested Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average remaining contractual term (in years)	4 years 10 months 21 days	4 years 8 months 12 days
Aggregate intrinsic value	$ 231,277	$ 87,286
Unvested Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding (in shares)	3,824,822	3,857,744
Weighted-average exercise price (in dollars per share)	$ 61.10	$ 50.97
Weighted-average remaining contractual term (in years)	6 years 4 months 2 days	6 years 1 month 10 days
Aggregate intrinsic value	$ 113,246	$ 23,891