Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
Deferred tax asset, valuation allowance	$ 46,578,000	$ 42,236,000
Deferred tax asset, valuation allowance increase (decrease)	$ 4,300,000	(13,800,000)
Number of years cumulative pre-tax loss	3 years
Unrecognized tax benefits	$ 4,334,000	$ 4,440,000	$ 9,331,000
Uncertain tax benefits that, if realized, would affect the effective tax rate	3,900,000
Accrued interest and penalties	0
Operating loss carryforwards state	106,700,000
Operating loss carryforwards federal	$ 172,200,000
Federal and state net operating loss carryforwards, expiring year	2038
Federal research and development tax credit carryforwards	$ 3,400,000
State research and development tax credit carryforwards	$ 3,100,000
Federal statutory income tax rate	21.00%	35.00%
Federal research and development tax credits [Member]
Income Tax [Line Items]
Federal research and development tax credits expiration year	2038