Employee Benefit Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Participants contribution	$ 18,000
Employer contribution	2.00%
Employer contribution	20.00%
Initial vesting period of employer contribution	2 years
Full vesting period of employer contribution	6 years
Contribution expenses	$ 7,900,000	$ 6,300,000	$ 5,400,000