Reconciliation of Effective Tax Rate for the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal statutory income tax rate	21.00%	34.00%	34.00%
Federal and state credits	7.40%	8.90%	9.50%
Excess tax benefit	0.40%	8.60%	0.00%
Stock based compensation	(0.80%)	0.10%	(0.10%)
Other	(0.60%)	(0.10%)	0.10%
Tax impact due to tax rate reduction	0.00%	(47.70%)	0.00%
Change in valuation allowance	(24.10%)	2.60%	(38.60%)
Foreign Rate Differential	(3.30%)	(6.40%)	(4.90%)
Total tax benefit	0.00%	0.00%	0.00%