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Revenue (Narrative) (Details)
$ in Millions
3 Months Ended
Mar. 31, 2019
USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Advanced billings, revenue recognized $ 80
Expected timing of satisfaction period 12 months
Support services | Minimum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Period of recognition 1 year
Support services | Maximum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Period of recognition 5 years
Business services | Minimum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Period of recognition 1 year
Business services | Maximum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Period of recognition 5 years