Basis of Presentation and Summary of Significant Accounting Policies (Changes in Estimates on Contracts) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Apr. 03, 2020	Mar. 29, 2019
Accounting Policies [Abstract]
Favorable impact	$ 24	$ 23
Unfavorable impact	(7)	(19)
Net impact to income before income taxes	$ 17	$ 4
Impact on diluted EPS attributable to Leidos common stockholders (usd per share)	$ 0.09	$ 0.02