Revenue (Revenue by Contract Type) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 5,639	$ 4,913	$ 4,171
Fixed Price
Disaggregation of Revenue [Line Items]
Revenues	1,719	1,508
Cost Reimbursable
Disaggregation of Revenue [Line Items]
Revenues	3,920	3,405
Operating Segments | Government Solutions
Disaggregation of Revenue [Line Items]
Revenues	3,925	3,457	2,193
Operating Segments | Government Solutions | Fixed Price
Disaggregation of Revenue [Line Items]
Revenues	1,111	1,031
Operating Segments | Government Solutions | Cost Reimbursable
Disaggregation of Revenue [Line Items]
Revenues	2,814	2,426
Operating Segments | Technology Solutions
Disaggregation of Revenue [Line Items]
Revenues	374	297	269
Operating Segments | Technology Solutions | Fixed Price
Disaggregation of Revenue [Line Items]
Revenues	367	288
Operating Segments | Technology Solutions | Cost Reimbursable
Disaggregation of Revenue [Line Items]
Revenues	7	9
Operating Segments | Energy Solutions
Disaggregation of Revenue [Line Items]
Revenues	1,339	1,157	1,671
Operating Segments | Energy Solutions | Fixed Price
Disaggregation of Revenue [Line Items]
Revenues	240	187
Operating Segments | Energy Solutions | Cost Reimbursable
Disaggregation of Revenue [Line Items]
Revenues	1,099	970
Non-strategic Business
Disaggregation of Revenue [Line Items]
Revenues	1	2	$ 38
Non-strategic Business | Fixed Price
Disaggregation of Revenue [Line Items]
Revenues	1	2
Non-strategic Business | Cost Reimbursable
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0