Basis of Presentation and Summary of Significant Accounting Policies (Changes in Estimates on Contracts) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jul. 03, 2020	Jun. 28, 2019	Jul. 03, 2020	Jun. 28, 2019
Accounting Policies [Abstract]
Favorable impact	$ 28	$ 23	$ 52	$ 46
Unfavorable impact	(19)	(10)	(26)	(29)
Net impact to income before income taxes	$ 9	$ 13	$ 26	$ 17
Impact on diluted EPS attributable to Leidos common stockholders (usd per share)	$ 0.05	$ 0.07	$ 0.14	$ 0.09