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Segment Information (Details) $ in Millions	3 Months Ended								5 Months Ended	12 Months Ended
Segment Information (Details) $ in Millions	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)	Oct. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment										2
Revenue	$ 1,094	$ 1,075	$ 1,068	$ 793	$ 715	$ 722	$ 706	$ 676	$ 1,073	$ 4,030	$ 2,819	$ 2,732
Defense and Intelligence
Segment Reporting Information [Line Items]
Revenue										2,587
Civilian and Health Care
Segment Reporting Information [Line Items]
Revenue										1,443
Operating segments
Segment Reporting Information [Line Items]
Revenue									1,073	4,030	2,819	2,732
Segment Profit									110	527	390	234
Depreciation and Amortization									73	330	167	225
Operating segments \| Series of Individually Immaterial Business Acquisitions
Segment Reporting Information [Line Items]
Depreciation and Amortization									0	165	69	0
Operating segments \| Defense and Intelligence
Segment Reporting Information [Line Items]
Revenue									488	2,587	1,416	1,153
Segment Profit									35	331	168	51
Depreciation and Amortization									34	74	34	95
Operating segments \| Civilian and Health Care
Segment Reporting Information [Line Items]
Revenue									585	1,443	1,403	1,579
Segment Profit									75	196	222	183
Depreciation and Amortization									$ 39	$ 91	$ 64	$ 130