New Accounting Standards Implemented - Impact of Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 30, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Contract assets	$ 1,499		$ 1,349	$ 0
Contracts in process	0		0	1,933
Inventories	994		926	389
Prepaid expenses and other current assets	392		317	300
Assets held for sale	136		138	135
Assets of discontinued operations	287		285	306
Total current assets	4,480		4,400	4,448
Other assets	346		313	264
Total assets	12,873		12,730	12,729
Liabilities
Accrued expenses	197		204	217
Contract liabilities	607		565	0
Advance payments and billings in excess of costs incurred	0		0	509
Other current liabilities	319		318	367
Liabilities held for sale	19		16	17
Liabilities of discontinued operations	154		203	226
Total current liabilities	2,335		2,349	2,379
Deferred income taxes	177		162	158
Other liabilities	397		412	398
Total liabilities	7,553		7,566	7,578
Shareholders' Equity
Retained earnings	6,814		6,672	6,659
Total equity	5,320	$ 4,802	5,164	5,151	$ 4,624
Net Sales:
Products	1,646	1,604
Services	725	717
Total net sales	2,371	2,321
Operating costs and expenses:
Cost of sales — Products	(1,191)	(1,188)
Cost of sales — Services	(532)	(538)
General and administrative expenses	(397)	(358)
Operating income	251	237
Income from continuing operations before income taxes	216	199
Provision for income taxes	(24)	(42)
Income from continuing operations	192	157
Income from discontinued operations, net of income taxes	16	11
Net income	208	168
Net income attributable to L3	$ 203	$ 164
Basic earnings per share attributable to common shareholders:
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
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	2.34	1.93
Discontinued operations (in dollars per share)	0.20	0.14
Diluted earnings per share (in dollars per share)	$ 2.54	$ 2.07
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Contract assets	$ 0			0
Contracts in process	2,124			1,933
Inventories	424			389
Prepaid expenses and other current assets	377			300
Assets held for sale				135
Assets of discontinued operations	312			306
Total current assets	4,545			4,448
Other assets	296			264
Total assets	12,888			12,729
Liabilities
Accrued expenses	218			217
Contract liabilities	0			0
Advance payments and billings in excess of costs incurred	576			509
Other current liabilities	367			367
Liabilities held for sale	20			17
Liabilities of discontinued operations	181			226
Total current liabilities	2,401			2,379
Deferred income taxes	169			158
Other liabilities	382			398
Total liabilities	7,596			7,578
Shareholders' Equity
Retained earnings	6,786			6,659
Total equity	5,292			$ 5,151
Net Sales:
Products	1,574
Services	721
Total net sales	2,295
Operating costs and expenses:
Cost of sales — Products	(1,145)
Cost of sales — Services	(528)
General and administrative expenses	(390)
Operating income	232
Income from continuing operations before income taxes	197
Provision for income taxes	(19)
Income from continuing operations	178
Income from discontinued operations, net of income taxes	15
Net income	193
Net income attributable to L3	$ 188
Basic earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	$ 2.21
Discontinued operations (in dollars per share)	0.19
Basic earnings per share (in dollars per share)	2.40
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	2.16
Discontinued operations (in dollars per share)	0.19
Diluted earnings per share (in dollars per share)	$ 2.35
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Assets
Contract assets	$ 1,499		1,349
Contracts in process	(2,124)		(1,933)
Inventories	570		537
Prepaid expenses and other current assets	15		17
Assets held for sale			3
Assets of discontinued operations	(25)		(21)
Total current assets	(65)		(48)
Other assets	50		49
Total assets	(15)		1
Liabilities
Accrued expenses	(21)		(13)
Contract liabilities	607		565
Advance payments and billings in excess of costs incurred	(576)		(509)
Other current liabilities	(48)		(49)
Liabilities held for sale	(1)		(1)
Liabilities of discontinued operations	(27)		(23)
Total current liabilities	(66)		(30)
Deferred income taxes	8		4
Other liabilities	15		14
Total liabilities	(43)		(12)
Shareholders' Equity
Retained earnings	28		13
Total equity	28		$ 13
Net Sales:
Products	72
Services	4
Total net sales	76
Operating costs and expenses:
Cost of sales — Products	(46)
Cost of sales — Services	(4)
General and administrative expenses	(7)
Operating income	19
Income from continuing operations before income taxes	19
Provision for income taxes	(5)
Income from continuing operations	14
Income from discontinued operations, net of income taxes	1
Net income	15
Net income attributable to L3	$ 15
Basic earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	$ 0.19
Discontinued operations (in dollars per share)	0.01
Basic earnings per share (in dollars per share)	0.20
Diluted earnings per share attributable to common shareholders:
Continuing operations (in dollars per share)	0.18
Discontinued operations (in dollars per share)	0.01
Diluted earnings per share (in dollars per share)	$ 0.19
Sales	$ 76
Electronic Systems [Member]
Net Sales:
Total net sales	785
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	3
Diluted earnings per share attributable to common shareholders:
Sales	30
Aerospace Systems [Member]
Net Sales:
Total net sales	686
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	1
Diluted earnings per share attributable to common shareholders:
Sales	4
Communication Systems [Member]
Net Sales:
Total net sales	493
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	4
Diluted earnings per share attributable to common shareholders:
Sales	20
Sensor Systems [Member]
Net Sales:
Total net sales	407
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	11
Diluted earnings per share attributable to common shareholders:
Sales	$ 22