Revenue - Schedule of Revenues Disaggregated By Contract Type (Details) - USD ($) $ in Millions	3 Months Ended								5 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Oct. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 1,094	$ 1,075	$ 1,068	$ 793	$ 715	$ 722	$ 706	$ 676	$ 1,073	$ 4,030	$ 2,819	$ 2,732
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues										920
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues										2,281
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues										829
Defense and Intelligence
Disaggregation of Revenue [Line Items]
Revenues										2,587
Defense and Intelligence | Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues										830
Defense and Intelligence | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues										1,365
Defense and Intelligence | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues										392
Civilian and Health Care
Disaggregation of Revenue [Line Items]
Revenues										1,443
Civilian and Health Care | Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues										90
Civilian and Health Care | Fixed-price
Disaggregation of Revenue [Line Items]
Revenues										916
Civilian and Health Care | Time-and-materials
Disaggregation of Revenue [Line Items]
Revenues										$ 437