Income Taxes - Variation Between Expected and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Statutory federal income tax rate	21.00%	35.00%	35.00%
State income taxes (net of federal benefit)	5.30%	2.50%	5.70%
Qualified domestic production activities	(0.00%)	(2.60%)	(2.00%)
Stock-based compensation expense	(1.90%)	(8.20%)	(0.00%)
Research and development tax credits	(32.00%)	(50.60%)	(8.60%)
Other tax credits	(1.20%)	(7.50%)	(0.00%)
Changes in valuation allowance	0.70%	10.60%	0.90%
Non-deductible book expenses	8.20%	1.10%	0.20%
Changes in deferred tax assets	12.10%	15.40%	1.50%
Re-measurement of deferred taxes for 2017 Tax Act	0.00%	(171.30%)	0.00%
Changes in tax reserves	1.20%	11.40%	0.00%
Other	(1.40%)	0.40%	1.00%
Effective income tax (benefit) rate	12.00%	(163.80%)	33.70%