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Revenue Recognition - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
United States      
Concentration Risk [Line Items]      
Concentration risk, percentage 88.80% 87.50%  
ASC 606      
Concentration Risk [Line Items]      
Accounts receivable $ 27.1   $ 19.5