New Accounting Standards Implemented - Impact of Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2018	Jun. 30, 2017	Jun. 29, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Contract assets	$ 1,611		$ 1,611		$ 1,349	$ 0
Contracts in process	0		0		0	1,933
Inventories	934		934		926	389
Prepaid expenses and other current assets	332		332		317	300
Assets held for sale	0		0		138	135
Assets of discontinued operations	0		0		285	306
Total current assets	5,106		5,106		4,400	4,448
Other assets	348		348		313	264
Total assets	13,530		13,530		12,730	12,729
Liabilities
Accrued expenses	264		264		204	217
Contract liabilities	554		554		565	0
Advance payments and billings in excess of costs incurred	0		0		0	509
Other current liabilities	338		338		318	367
Liabilities held for sale	0		0		16	17
Liabilities of discontinued operations	0		0		203	226
Total current liabilities	2,803		2,803		2,349	2,379
Deferred income taxes	190		190		162	158
Other liabilities	416		416		412	398
Total liabilities	8,018		8,018		7,566	7,578
Shareholders' Equity
Retained earnings	7,126		7,126		6,672	6,659
Total equity	5,512	$ 5,047	5,512	$ 5,047	5,164	5,151	$ 4,624
Net Sales:
Products	1,786	1,680	3,432	3,284
Services	797	705	1,522	1,422
Total net sales	2,583	2,385	4,954	4,706
Operating costs and expenses:
Cost of sales — Products	(1,349)	(1,253)	(2,540)	(2,441)
Cost of sales — Services	(578)	(479)	(1,110)	(1,017)
General and administrative expenses	(383)	(359)	(780)	(717)
Gain on sale of the Crestview Aerospace and TCS businesses	48	0	48	0
Operating income	321	294	572	531
Income from continuing operations before income taxes	237	254	453	453
Provision for income taxes	(48)	(59)	(72)	(101)
Income from continuing operations	189	195	381	352
Income from discontinued operations, net of income taxes	190	12	206	23
Net income	379	207	587	375
Net income attributable to L3	$ 375	$ 202	$ 578	$ 366
Basic earnings per share attributable to common shareholders:
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
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	2.33	2.39	4.67	4.32
Discontinued operations (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
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Contract assets	$ 0		$ 0			0
Contracts in process	2,185		2,185			1,933
Inventories	405		405			389
Prepaid expenses and other current assets	316		316			300
Assets held for sale	0		0			135
Assets of discontinued operations	0		0			306
Total current assets	5,135		5,135			4,448
Other assets	300		300			264
Total assets	13,511		13,511			12,729
Liabilities
Accrued expenses	279		279			217
Contract liabilities	0		0			0
Advance payments and billings in excess of costs incurred	522		522			509
Other current liabilities	384		384			367
Liabilities held for sale						17
Liabilities of discontinued operations						226
Total current liabilities	2,832		2,832			2,379
Deferred income taxes	181		181			158
Other liabilities	402		402			398
Total liabilities	8,024		8,024			7,578
Shareholders' Equity
Retained earnings	7,101		7,101			6,659
Total equity	5,487		5,487			5,151
Net Sales:
Products	1,741		3,315
Services	797		1,518
Total net sales	2,538		4,833
Operating costs and expenses:
Cost of sales — Products	(1,316)		(2,461)
Cost of sales — Services	(561)		(1,089)
General and administrative expenses	(387)		(777)
Gain on sale of the Crestview Aerospace and TCS businesses	47		47
Operating income	321		553
Income from continuing operations before income taxes	237		434
Provision for income taxes	(48)		(67)
Income from continuing operations	189		367
Income from discontinued operations, net of income taxes	191		206
Net income	380		573
Net income attributable to L3	$ 376		$ 564
Basic earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	$ 2.36		$ 4.57
Discontinued operations (in dollars per share)	2.44		2.63
Basic earnings per share (in dollars per share)	4.80		7.20
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	2.33		4.50
Discontinued operations (in dollars per share)	2.41		2.59
Diluted earnings per share (in dollars per share)	$ 4.74		$ 7.09
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Assets
Contract assets	$ 1,611		$ 1,611		1,349
Contracts in process	(2,185)		(2,185)		(1,933)
Inventories	529		529		537
Prepaid expenses and other current assets	16		16		17
Assets held for sale	0		0		3
Assets of discontinued operations	0		0		(21)
Total current assets	(29)		(29)		(48)
Other assets	48		48		49
Total assets	19		19		1
Liabilities
Accrued expenses	(15)		(15)		(13)
Contract liabilities	554		554		565
Advance payments and billings in excess of costs incurred	(522)		(522)		(509)
Other current liabilities	(46)		(46)		(49)
Liabilities held for sale					(1)
Liabilities of discontinued operations					(23)
Total current liabilities	(29)		(29)		(30)
Deferred income taxes	9		9		4
Other liabilities	14		14		14
Total liabilities	(6)		(6)		(12)
Shareholders' Equity
Retained earnings	25		25		13
Total equity	25		25		$ 13
Net Sales:
Products	45		117
Services	0		4
Total net sales	45		121
Operating costs and expenses:
Cost of sales — Products	(33)		(79)
Cost of sales — Services	(17)		(21)
General and administrative expenses	4		(3)
Gain on sale of the Crestview Aerospace and TCS businesses	1		1
Operating income	0		19
Income from continuing operations before income taxes	0		19
Provision for income taxes	0		(5)
Income from continuing operations	0		14
Income from discontinued operations, net of income taxes	(1)		0
Net income	(1)		14
Net income attributable to L3	$ (1)		$ 14
Basic earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	$ 0.00		$ 0.18
Discontinued operations (in dollars per share)	(0.02)		0.00
Basic earnings per share (in dollars per share)	(0.02)		0.18
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	0.00		0.17
Discontinued operations (in dollars per share)	(0.02)		0.00
Diluted earnings per share (in dollars per share)	$ (0.02)		$ 0.17
Electronic Systems [Member]
Net Sales:
Total net sales	$ 812		$ 1,597
Electronic Systems [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating costs and expenses:
Operating income	4		7
Diluted earnings per share attributable to common shareholders:
Sales	14		44
Aerospace Systems [Member]
Net Sales:
Total net sales	741		1,427
Aerospace Systems [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating costs and expenses:
Operating income	(7)		(6)
Diluted earnings per share attributable to common shareholders:
Sales	(2)		2
Communication Systems [Member]
Net Sales:
Total net sales	554		1,047
Communication Systems [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating costs and expenses:
Operating income	(1)		3
Diluted earnings per share attributable to common shareholders:
Sales	21		41
Sensor Systems [Member]
Net Sales:
Total net sales	476		883
Sensor Systems [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating costs and expenses:
Operating income	3		14
Diluted earnings per share attributable to common shareholders:
Sales	12		34
Operating Segments [Member]
Operating costs and expenses:
Operating income	273	$ 294	524	$ 531
Operating Segments [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Operating costs and expenses:
Operating income	(1)		18
Diluted earnings per share attributable to common shareholders:
Sales	45		121
Operating Segments [Member] | Electronic Systems [Member]
Assets
Total assets	4,990		4,990			4,796
Net Sales:
Total net sales	828	789	1,630	1,549
Operating costs and expenses:
Operating income	113	103	221	193
Operating Segments [Member] | Aerospace Systems [Member]
Assets
Total assets	2,200		2,200			2,014
Net Sales:
Total net sales	751	681	1,450	1,382
Operating costs and expenses:
Operating income	61	56	118	112
Operating Segments [Member] | Communication Systems [Member]
Assets
Total assets	2,142		2,142			2,063
Net Sales:
Total net sales	563	555	1,062	1,098
Operating costs and expenses:
Operating income	45	84	82	126
Operating Segments [Member] | Sensor Systems [Member]
Assets
Total assets	2,890		2,890			$ 2,706
Net Sales:
Total net sales	482	394	894	748
Operating costs and expenses:
Operating income	$ 54	$ 51	$ 103	$ 100