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, 2016		Sep. 23, 2016		Jun. 24, 2016		Mar. 25, 2016		Dec. 31, 2015		Sep. 25, 2015		Jun. 26, 2015		Mar. 27, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of net income (loss) [Abstract]
Net income (loss)		$ 192		$ 151		$ 151		$ 230		$ (162)		$ (296)		$ 124		$ 109		$ 724	$ (225)	$ 677
Net income from continuing operations attributable to noncontrolling interests																		(14)	(15)	(13)
Net income (loss) attributable to L3		$ 188		$ 148		$ 147		$ 227		$ (166)		$ (299)		$ 120		$ 105		710	(240)	664
Earnings (loss) attributable to L3's common shareholders: [Abstract]
Continuing operations																		647	282	632
Discontinued operations, net of income tax																		63	(522)	32
Net income (loss) attributable to L3 common shareholders																		$ 710	$ (240)	$ 664
Basic [Abstract]
Weighted average common shares outstanding (in shares)																		77.4	80.7	85.4
Basic earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)		$ 2.43	[1]	$ 1.91	[1]	$ 1.90	[1]	$ 2.11	[1]	$ (0.76)	[1]	$ 1.56	[1]	$ 1.41	[1]	$ 1.23	[1]	$ 8.36	$ 3.49	$ 7.40
Discontinued operations, net of income tax (in dollars per share)																		0.81	(6.46)	0.38
Basic earnings (loss) per share (in dollars per share)		2.43	[1]	1.91	[1]	1.90	[1]	2.92	[1]	(2.11)	[1]	(3.74)	[1]	1.46	[1]	1.28	[1]	$ 9.17	$ (2.97)	$ 7.78
Common and potential common shares [Abstract]
Weighted average common shares outstanding (in shares)																		77.4	80.7	85.4
Assumed purchase of common shares for treasury (in shares)																		(2.4)	(2.7)	(3.1)
Assumed conversion of the CODES (in shares)	[2]																	0.0	0.0	0.8
Common and potential common shares (in shares)																		78.8	81.9	87.8
Diluted earnings (loss) per share [Abstract]
Continuing operations (in dollars per share)		2.38	[1]	1.88	[1]	1.88	[1]	2.08	[1]	(0.76)	[1]	1.54	[1]	1.39	[1]	1.20	[1]	$ 8.21	$ 3.44	$ 7.20
Discontinued operations, net of income tax (in dollars per share)																		0.80	(6.37)	0.36
Diluted earnings (loss) per share (in dollars per share)		$ 2.38	[1]	$ 1.88	[1]	$ 1.88	[1]	$ 2.87	[1]	$ (2.11)	[1]	$ (3.68)	[1]	$ 1.44	[1]	$ 1.25	[1]	$ 9.01	$ (2.93)	$ 7.56
Stock Options [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)																		2.6	2.4	3.0
Unvested Restricted Stock Awards [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)																		1.0	1.3	1.6
Performance Unit Awards [Member]
Common and potential common shares [Abstract]
Common and potential common shares (in shares)																		0.1	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	0.0

[1] EPS in each quarter is computed using the weighted-average number of shares outstanding during that quarter, while EPS for the full year is computed using the weighted-average number of shares outstanding during the year. Therefore, the sum of the four quarters' EPS may not equal the full year computed EPS.

[2] The CODES were retired on June 20, 2014 and were dilutive for the year ended December 31, 2014 as the average market price of L3's common stock during the period that the CODES were outstanding was greater than the price at which the CODES would have been convertible into L3's common stock. As of June 18, 2014, the final date of conversion, the conversion price was $88.71.