Income Taxes - Reconciliation of Effective Tax Rate for the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
Federal and state credits	3.20%	7.40%	8.90%
Excess tax benefit	0.30%	0.40%	8.60%
Stock based compensation	(2.50%)	(0.80%)	0.10%
U.S. tax on foreign earnings	(0.20%)	0.00%	0.00%
Other	(0.50%)	(0.60%)	(0.10%)
Tax impact due to tax rate reduction	0.00%	0.00%	(47.70%)
Change in valuation allowance	(19.80%)	(24.10%)	2.60%
Foreign rate differential	(1.50%)	(3.30%)	(6.40%)
Total tax provision	0.00%	0.00%	0.00%