L3's Earnings Per Share, Reconciliation of Basic and Diluted Earnings Per Share (EPS) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 28, 2018	Jun. 29, 2018	Mar. 30, 2018	Dec. 31, 2017	Sep. 29, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of net income (loss) [Abstract]
Net income	$ 231	$ 208	$ 379	$ 208	$ 293	$ 25	$ 207	$ 168	$ 1,026	$ 693	$ 724
Net income from continuing operations attributable to noncontrolling interests									(21)	(16)	(14)
Net income attributable to L3	225	202	375	203	289	22	202	164	1,005	677	710
Earnings (loss) attributable to L3's common shareholders: [Abstract]
Continuing operations									800	753	619
Discontinued operations, net of income tax									205	(76)	91
Net income attributable to L3	$ 225	$ 202	$ 375	$ 203	$ 289	$ 22	$ 202	$ 164	$ 1,005	$ 677	$ 710
Basic [Abstract]
Weighted average common shares outstanding (in shares)									78.5	78.0	77.4
Basic earnings (loss) per share attributable to common shareholders:
Continuing operations (in dollars per share)	$ 2.87	$ 2.57	$ 2.36	$ 2.40	$ 3.42	$ 1.83	$ 2.44	$ 1.97	$ 10.19	$ 9.65	$ 7.99
Discontinued operations, net of income tax (in dollars per share)									2.61	(0.97)	1.18
Basic earnings (loss) per share (in dollars per share)	2.86	2.57	4.78	2.60	3.70	0.28	2.59	2.11	$ 12.80	$ 8.68	$ 9.17
Common and potential common shares [Abstract]
Weighted average common shares outstanding (in shares)									78.5	78.0	77.4
Effect of dilutive securities									1.1	1.6	1.4
Common and potential common shares (in shares)									79.6	79.6	78.8
Diluted earnings (loss) per share attributable to common shareholders:
Continuing operations (in dollars per share)	2.83	2.54	2.33	2.34	3.34	1.79	2.39	1.93	$ 10.05	$ 9.46	$ 7.86
Discontinued operations, net of income tax (in dollars per share)									2.58	(0.95)	1.15
Diluted earnings (loss) per share (in dollars per share)	$ 2.82	$ 2.54	$ 4.72	$ 2.54	$ 3.62	$ 0.28	$ 2.54	$ 2.07	$ 12.63	$ 8.51	$ 9.01