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Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Property and Equipment    
Depreciation $ 0.5 $ 0.5
Leasehold Improvements    
Property and Equipment    
Useful life (in years) 10 years  
Minimum    
Property and Equipment    
Useful life (in years) 3 years  
Maximum    
Property and Equipment    
Useful life (in years) 10 years