Federal Income Tax Provision Compared With Statutory Rates (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Statutory regular federal income tax rate	(34.00%)	(34.00%)	(34.00%)
Change in valuation allowance	(90.60%)	40.40%	40.20%
State tax benefit (net of federal benefit)	(3.20%)	(3.00%)	(3.10%)
Research credits	(1.70%)	(3.40%)	(3.10%)
Foreign amounts with no tax benefit		0.20%	0.10%
Non-deductible expenses	1.00%	0.60%	0.40%
Effect of change in rate from federal Tax Reform	127.10%	0.50%	0.90%
Other	(2.00%)	(0.20%)	(0.50%)
Total	(3.40%)	1.10%	0.90%