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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Tax provision derived by applying the federal statutory rate to income before income taxes	$ (9,260,000)	$ (7,377,000)	$ (3,841,000)
Permanent differences and other	296,000	333,000	307,000
Change in tax rate	7,869,000
Conversion of LLC from partnership to corporation			(21,000)
Change in the valuation allowance	2,673,000	9,369,000	3,876,000
Income tax expense /(benefit)	0	0	0
Federal
Income Taxes [Line Items]
Tax credits	(1,294,000)	(1,921,000)	$ (321,000)
State
Income Taxes [Line Items]
Tax credits	$ (284,000)	$ (404,000)