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Revenue (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Revenue from Contract with Customer [Abstract]    
Revenue recognized from performance obligation satisfied in previous period $ 8 $ 1
Disaggregation of Revenue [Line Items]    
Revenues 1,340 1,038
Government Solutions    
Disaggregation of Revenue [Line Items]    
Revenues 975 677
Government Solutions | Key U.S. Government Customers    
Disaggregation of Revenue [Line Items]    
Revenues 763 492
Government Solutions | Non U.S. Government Customers    
Disaggregation of Revenue [Line Items]    
Revenues $ 212 $ 185