Income Taxes - Reconciliation of Federal Income Tax Rate to Losses Before Income Tax Benefit (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax benefit at federal statutory rate	$ (2,661,000)	$ (12,451,000)
State income taxes, net of federal benefit	(570,000)	(751,000)
Non-deductible expenses	154,000	235,000
Federal rate impact	0	18,894,000
Change in fair value of warrants	(3,479,000)	0
Research and development tax credits	(1,254,000)	(1,732,000)
Other	380,000	431,000
Change in valuation allowance	7,430,000	(4,626,000)
Total income tax provision	$ 0	$ 0