Postretirement Plans - Actuarial Assumptions (Narrative) (Details) - Vencore Holding Corp. and KGS Holding Corp. - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	6.50%	6.50%
Benefit per participant per quarter		$ 25
Reduction to interest cost	$ 2,600,000	$ 3,600,000

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Period Type:	duration