Income Taxes - Schedule of Reconciliation of the Company's Effective Tax Rate to the Statutory Federal Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax at statutory federal rate	35.00%	35.00%	35.00%
State taxes	1.70%	0.60%	1.20%
Permanent differences	(3.00%)	(6.30%)	(2.40%)
Stock-based compensation	(7.20%)	(1.50%)	(1.00%)
Tax Cuts and Jobs Act impact	(19.90%)
Tax credits	11.60%	12.60%	9.00%
Change in deferred state tax rate	7.70%	(0.30%)	(1.90%)
Other	0.30%	3.70%	(0.50%)
Change in valuation allowance	9.90%	(36.00%)	(32.50%)
Total	36.10%	7.80%	6.90%