Income Taxes - Schedule of Reconciliation of Income Taxes Computed Using U.S. Federal Statutory Rate to that Reflected in Operations (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes using U.S. federal statutory rate	21.00%	34.00%	34.00%
State income taxes, net of federal benefit	6.82%	5.33%	5.44%
Tax Cuts and Jobs Act	0.00%	(44.43%)	0.00%
Impact of R&D tax credit on effective tax rate	3.48%	3.25%	3.24%
Stock option shortfalls and cancellations	(0.43%)	0.21%	(0.07%)
Permanent items and other	(0.15%)	(0.56%)	(0.64%)
Change in valuation allowance	(31.76%)	2.55%	(41.17%)
Provision to return	0.03%	0.00%	0.00%
Non-taxable revenue	1.54%	0.00%	0.00%
Total	0.53%	0.35%	0.80%