Basis of Presentation and Summary of Significant Accounting Policies (Changes in Estimates on Contracts) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Oct. 02, 2020	Sep. 27, 2019	Oct. 02, 2020	Sep. 27, 2019
Accounting Policies [Abstract]
Favorable impact	$ 34	$ 23	$ 86	$ 69
Unfavorable impact	(19)	(11)	(45)	(40)
Net impact to income before income taxes	$ 15	$ 12	$ 41	$ 29
Impact on diluted EPS attributable to Leidos common stockholders (usd per share)	$ 0.07	$ 0.06	$ 0.21	$ 0.15