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