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Summary of Significant Accounting Policies - Software Development Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Software Development Costs      
Software development costs capitalized $ 0 $ 0 $ 0
Software development | Minimum      
Software Development Costs      
Estimated useful lives of assets 2 years    
Software development | Maximum      
Software Development Costs      
Estimated useful lives of assets 5 years