Income Taxes - Difference Between Provision for Income Taxes and Amounts Computed by Applying Statutory Federal Income Tax Rate to Income Before Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Tax provision derived by applying the federal statutory rate to income before income taxes	$ (9,313,000)	$ (9,260,000)	$ (7,377,000)
Permanent differences and other	264,000	296,000	333,000
Change in tax rate	0	7,869,000
Change in the valuation allowance	9,796,000	2,673,000	9,369,000
Income tax expense /(benefit)	0	0	0
Federal
Income Taxes [Line Items]
Tax credits	(1,211,000)	(1,294,000)	(1,921,000)
State
Income Taxes [Line Items]
Tax credits	$ 464,000	$ (284,000)	$ (404,000)