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, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Tax provision derived by applying the federal statutory rate to income before income taxes	$ (16,988,000)	$ (16,433,000)	$ (9,313,000)
Permanent differences and other	482,000	(98,000)	264,000
Change in the valuation allowance	20,662,000	20,359,000	9,796,000
Income tax expense /(benefit)	0	0	0
Federal
Income Taxes [Line Items]
Tax credits	(3,905,000)	(3,599,000)	(1,211,000)
State
Income Taxes [Line Items]
Tax credits	$ (251,000)	$ (229,000)	$ 464,000