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Income Taxes - Effective Tax Rate (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
Tax at federal statutory rate 21.00% 34.00% 34.00%
State tax, net of federal benefit 1.88% 1.03% 0.15%
Stock-based compensation (5.84%) (38.63%) (8.19%)
Warrant expense (6.98%) (1.00%) (0.03%)
Other items (1.46%) (1.10%) (0.66%)
Research and development credits 10.54% 102.35% 0.00%
Net operating loss expiration 0.00% (9.29%) 0.00%
Change in valuation allowance (19.21%) 458.55% (25.26%)
Rate differential impact of Tax Cuts and Jobs Act of 2017 0.00% (545.38%) 0.00%
Effective tax rate (0.07%) 0.53% 0.01%