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Income Taxes - Summary (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Taxes      
Corporate tax rate (as a percent) 21.00% 34.00% 34.00%
Federal | Net Operating Losses      
Income Taxes      
Net operating loss carryforwards $ 217.2 $ 200.3  
Indefinite net operating loss carryforward 16.9    
Federal | Research and Development      
Income Taxes      
Tax credit carryforwards 5.8    
State | Net Operating Losses      
Income Taxes      
Net operating loss carryforwards 78.1    
State | Research and Development      
Income Taxes      
Tax credit carryforwards $ 1.3