L3's Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 25, 2016
Reconciliation of net income [Abstract]
Net income	$ 168	$ 230
Net income from continuing operations attributable to noncontrolling interests	(4)	(3)
Net income attributable to L3	164	227
Earnings attributable to L3' s common shareholders [Abstract]
Continuing operations	164	164
Discontinued operations, net of income tax	0	63
Net income attributable to L3' s common shareholders	$ 164	$ 227
Basic [Abstract]
Weighted average common shares outstanding (in shares)	77.7	77.8
Basic earnings per share [Abstract]
Continuing operations (in dollars per share)	$ 2.11	$ 2.11
Discontinued operations, net of income tax (in dollars per share)	0	0.81
Basic earnings per share (in dollars per share)	$ 2.11	$ 2.92
Common and potential common shares [Abstract]
Weighted average common shares outstanding (in shares)	77.7	77.8
Assumed purchase of common shares for treasury (in shares)	(2.2)	(2.1)
Common and potential common shares (in shares)	79.3	79.0
Diluted earnings per share [Abstract]
Continuing operations (in dollars per share)	$ 2.07	$ 2.08
Discontinued operations, net of income tax (in dollars per share)	0	0.79
Diluted earnings per share (in dollars per share)	$ 2.07	$ 2.87
Shares excluded from computation of diluted earnings per share (in shares)	0.3	1.2
Stock Options [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)	2.8	2.0
Unvested Restricted Stock Awards [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)	0.8	1.1
Performance Unit Awards [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)	0.1	0.1
Employee Stock Purchase Plan Contributions [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)	0.1	0.1