Revenue - Schedule of Revenues Disaggregated By Contract Type (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 1,099	$ 1,126	$ 1,172	$ 1,107	$ 1,094	$ 1,075	$ 1,068	$ 793	$ 4,504	$ 4,030	$ 2,819
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues									1,207	920
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									2,439	2,281
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues									858	829
Defense and Intelligence
Disaggregation of Revenue [Line Items]
Revenues									3,101	2,587
Defense and Intelligence | Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues									1,100	830
Defense and Intelligence | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									1,602	1,365
Defense and Intelligence | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues									399	392
Civilian and Health Care
Disaggregation of Revenue [Line Items]
Revenues									1,403	1,443
Civilian and Health Care | Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues									107	90
Civilian and Health Care | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									837	916
Civilian and Health Care | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues									$ 459	$ 437