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Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Property and Equipment        
Depreciation $ 0.5 $ 0.4 $ 1.1 $ 0.9
Minimum        
Property and Equipment        
Useful life (in years)     3 years  
Maximum        
Property and Equipment        
Useful life (in years)     7 years