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Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Property and Equipment      
Depreciation $ 2.3 $ 1.8 $ 1.9
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