L3's Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 30, 2018	Mar. 31, 2017
Reconciliation of Net Income [Abstract]
Net income	$ 208	$ 168
Net income from continuing operations attributable to noncontrolling interests	(5)	(4)
Net income attributable to L3	203	164
Earnings attributable to L3' s common shareholders [Abstract]
Continuing operations	187	153
Discontinued operations, net of income tax	16	11
Net income attributable to L3	$ 203	$ 164
Basic [Abstract]
Weighted average common shares outstanding (in shares)	78.2	77.7
Basic earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)	$ 2.40	$ 1.97
Discontinued operations (in dollars per share)	0.20	0.14
Basic earnings per share (in dollars per share)	$ 2.60	$ 2.11
Common and potential common shares [Abstract]
Weighted average common shares outstanding (in shares)	78.2	77.7
Assumed Purchase Of Common Shares For Treasury	1.7	1.6
Common and potential common shares (in shares)	79.9	79.3
Diluted earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)	$ 2.34	$ 1.93
Discontinued operations, net of income tax (in dollars per share)	0.20	0.14
Diluted earnings per share (in dollars per share)	$ 2.54	$ 2.07
Shares excluded from computation of diluted earnings per share (in shares)		0.3