Employee Benefit Plan - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Participants contribution	$ 18,500
Employer contribution as a percentage of employee contributions	2.00%
Employer contribution	25.00%
Full vesting period of employer contribution	4 years
Contribution expenses	$ 10,100,000	$ 7,900,000	$ 6,300,000