Acquisitions - Pro Forma Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions			1 Months Ended	2 Months Ended	3 Months Ended					7 Months Ended	9 Months Ended
		May 31, 2018	Jun. 30, 2018	May 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition, Pro Forma Information [Abstract]
Revenue					$ 1,075		$ 722				$ 2,936	$ 2,104
Net income (loss)			$ (20)	$ 49	$ 38	$ 24	$ 104	$ 40	$ 32		$ 91	$ 176
Earnings per common share
Basic (in dollars per share)	[1]				$ 0.23		$ 0.73				$ 0.55	$ 1.24
Diluted (in dollars per share)	[1]				$ 0.23		$ 0.73				$ 0.55	$ 1.24
Weighted average common shares outstanding for basic EPS (in shares)		165,700			164,320		142,430				165,020	142,430
Weighted average common shares outstanding for diluted EPS (in shares)					164,540		142,430				165,270	142,430
Vencore and KeyPoint
Business Acquisition, Pro Forma Information [Abstract]
Revenue				245			$ 349					$ 1,041
Net income (loss)				$ (57)			20					22
Vencore and KeyPoint
Business Acquisition, Pro Forma Information [Abstract]
Revenue										$ 860
Net income (loss)										$ 137
Vencore and KeyPoint | Pro Forma Combined for the Spin-Off and Mergers
Business Acquisition, Pro Forma Information [Abstract]
Revenue							1,071				$ 3,181	3,145
Net income (loss)							$ 92				$ 39	$ 119
Earnings per common share
Basic (in dollars per share)							$ 0.56				$ 0.24	$ 0.72
Diluted (in dollars per share)							$ 0.56				$ 0.24	$ 0.72
Vencore and KeyPoint | Effects of the Spin-Off
Business Acquisition, Pro Forma Information [Abstract]
Revenue							$ 0				$ 0	$ 0
Net income (loss)							0				(7)	(18)
Vencore and KeyPoint | Effects of the Mergers
Business Acquisition, Pro Forma Information [Abstract]
Revenue							0				0	0
Net income (loss)							$ (32)				$ 12	$ (61)

[1] Earnings per share information for the three and nine months ended December 31, 2017 is computed using the 142.43 million shares of Perspecta common stock resulting from the Distribution, as Perspecta did not operate as a stand-alone entity during the period and therefore, no Perspecta common stock, stock options or other equity awards were outstanding and no dividends were declared or paid by Perspecta.