Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Benefit [Line Items]
Net operating loss carryforwards, federal	$ 128,200,000
Net operating loss carryforwards, state	$ 117,000,000
Net operating loss carryforwards, expiration year	2034
Net operating loss carryforwards, expiration date description	Begin to expire in 2034
Research and development tax credit	$ 11,766,000	$ 5,817,000
Research and development tax credits, expiration date description	Begin to expire in 2034 and 2029
Increase in valuation allowance	$ 28,700,000	$ 14,800,000	$ 13,100,000
Unrecognized tax benefits	$ 0
U.S. corporate income tax rate	21.00%	34.00%	34.00%
Provisional amount for decrease in net deferred tax assets offset by decrease in valuation allowance due to remeasurement		$ 12,600,000
Maximum [Member]
Income Tax Benefit [Line Items]
U.S. corporate income tax rate		35.00%
Measurement period		1 year
Federal [Member]
Income Tax Benefit [Line Items]
Research and development tax credit	$ 7,500,000
Research and development tax credits, expiration year	2034
State [Member]
Income Tax Benefit [Line Items]
Research and development tax credit	$ 4,800,000
Research and development tax credits, expiration year	2030