Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Income tax expense (benefit)		$ (97,000)	$ 168,000	$ 166,000
Undistributed earnings of foreign subsidiaries		0
Unrecognized deferred tax liability		17,000
Increase (decrease) in valuation allowance		(9,800,000)	905,000
Accumulated federal tax losses		126,400,000	100,000,000
Accumulated state tax losses		18,400,000	15,100,000
Research and development credit carry-forwards		$ 9,700,000	8,500,000
Federal tax credit carryforward expiration year		2023
Unrecognized tax benefits		$ 2,906,000	$ 2,597,000	$ 1,878,000	$ 1,732,000
Accrued interest and penalties related to unrecognized tax benefits		$ 0
US federal corporate tax rate		34.00%	34.00%	34.00%
Deferred tax assets, provisional amount		$ 19,800,000
Maximum
Income Tax Contingency [Line Items]
US federal corporate tax rate		35.00%
Scenario Plan
Income Tax Contingency [Line Items]
US federal corporate tax rate	21.00%