L3's Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017
Reconciliation of Net Income [Abstract]
Net income	$ 379	$ 207	$ 587	$ 375
Net income from continuing operations attributable to noncontrolling interests	(4)	(5)	(9)	(9)
Net income attributable to L3	375	202	578	366
Earnings attributable to L3' s common shareholders [Abstract]
Continuing operations	185	190	372	343
Discontinued operations, net of income tax	190	12	206	23
Net income attributable to L3	$ 375	$ 202	$ 578	$ 366
Basic [Abstract]
Weighted average common shares outstanding (in shares)	78.4	78.0	78.3	77.8
Basic earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)	$ 2.36	$ 2.44	$ 4.75	$ 4.40
Discontinued operations (in dollars per share)	2.42	0.15	2.63	0.30
Basic earnings per share (in dollars per share)	$ 4.78	$ 2.59	$ 7.38	$ 4.70
Common and potential common shares [Abstract]
Weighted average common shares outstanding (in shares)	78.4	78.0	78.3	77.8
Effect of dilutive securities (in shares)	1.0	1.5	1.3	1.6
Common and potential common shares (in shares)	79.4	79.5	79.6	79.4
Diluted earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)	$ 2.33	$ 2.39	$ 4.67	$ 4.32
Discontinued operations, net of income tax (in dollars per share)	2.39	0.15	2.59	0.29
Diluted earnings per share (in dollars per share)	$ 4.72	$ 2.54	$ 7.26	$ 4.61
Shares excluded from computation of diluted earnings per share (in shares)	0.3	0.4	0.3	0.3