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Revenue Recognition (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Segment Reporting Information [Line Items]      
Concentration risk, percentage 32.00% 31.00% 30.00%
Concentration risk second customer 10.00%    
Deferred Revenue, Revenue Recognized $ 690.1