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Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Property and Equipment        
Depreciation $ 0.6 $ 0.4 $ 1.7 $ 1.3
Minimum        
Property and Equipment        
Useful life (in years)     3 years  
Maximum        
Property and Equipment        
Useful life (in years)     7 years