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Stock-Based Compensation (Total Stock-Based Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Stock-based compensation expense $ 31.7 $ 24.1 $ 14.5
Cost of revenue [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Stock-based compensation expense 11.7 9.6 7.5
Selling and marketing expense [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Stock-based compensation expense 3.5 3.0 1.9
General and administrative expense [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Stock-based compensation expense $ 16.5 $ 11.5 $ 5.1