Summary of Significant Accounting Policies Statement of operations items (Details) - USD ($) $ / shares in Units, $ 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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	$ 2,771	$ 2,519	$ 2,583	$ 2,371	$ 2,574	$ 2,293	$ 2,385	$ 2,321	$ 10,244	$ 9,573	$ 9,210
Cost of sales									(9,138)	(8,542)	(8,252)
General and administrative expenses									(1,601)	(1,520)	(1,287)
Gain on sale of the Crestview Aerospace and TCS businesses									42	0	0
Merger and acquisition related expenses									(28)	0	0
Operating Income (Loss)	276	272	321	251	268	232	294	237	1,120	1,031	958
Income from continuing operations before income taxes									924	871	804
Provision for income taxes									(103)	(102)	(171)
Income from continuing operations	232	208	189	192	271	146	195	157	821	769	633
Income (loss) from discontinued operations, net of income taxes									205	(76)	91
Net (loss) income	231	208	379	208	293	25	207	168	1,026	693	724
Net income attributable to L3	$ 225	$ 202	$ 375	$ 203	$ 289	$ 22	$ 202	$ 164	$ 1,005	$ 677	$ 710
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 (in dollars per share)									2.61	(0.97)	1.18
Basic EPS (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
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 (in dollars per share)									2.58	(0.95)	1.15
Earnings Per Share, Diluted	$ 2.82	$ 2.54	$ 4.72	$ 2.54	$ 3.62	$ 0.28	$ 2.54	$ 2.07	$ 12.63	$ 8.51	$ 9.01
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 10,105
General and administrative expenses									(1,598)
Gain on sale of the Crestview Aerospace and TCS businesses									41
Merger and acquisition related expenses									(28)
Operating Income (Loss)									1,092
Income from continuing operations before income taxes									896
Provision for income taxes									(96)
Income from continuing operations									800
Income (loss) from discontinued operations, net of income taxes									214
Net (loss) income									1,014
Net income attributable to L3									$ 993
Continuing operations (in dollars per share)									$ 9.92
Discontinued operations (in dollars per share)									2.73
Basic EPS (in dollars per share)									12.65
Continuing operations (in dollars per share)									9.79
Discontinued operations (in dollars per share)									2.69
Earnings Per Share, Diluted									$ 12.48
Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 7,165	$ 6,733	$ 6,453
Cost of sales									(5,345)	(4,967)	(4,879)
Product [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									7,004
Cost of sales									(5,235)
Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									3,079	2,840	2,757
Cost of sales									(2,192)	(2,055)	(2,086)
Service [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									3,101
Cost of sales									(2,193)
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									139
General and administrative expenses									(3)
Gain on sale of the Crestview Aerospace and TCS businesses									1
Merger and acquisition related expenses									0
Operating Income (Loss)									28
Income from continuing operations before income taxes									28
Provision for income taxes									(7)
Income from continuing operations									21
Income (loss) from discontinued operations, net of income taxes									(9)
Net (loss) income									12
Net income attributable to L3									$ 12
Continuing operations (in dollars per share)									$ 0.27
Discontinued operations (in dollars per share)									(0.12)
Basic EPS (in dollars per share)									0.15
Continuing operations (in dollars per share)									0.26
Discontinued operations (in dollars per share)									(0.11)
Earnings Per Share, Diluted									$ 0.15
Accounting Standards Update 2014-09 [Member] | Product [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 161
Cost of sales									(110)
Accounting Standards Update 2014-09 [Member] | Service [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									(22)
Cost of sales									1
ISR Systems [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									4,441
ISR Systems [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,907
ISR Systems [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,534
ISR Systems [Member] | Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									21
Operating Income (Loss)									1
C&NS [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									3,059
C&NS [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,299
C&NS [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									760
C&NS [Member] | Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									68
Operating Income (Loss)									12
Electronic Systems [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,744
Electronic Systems [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,959
Electronic Systems [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									785
Electronic Systems [Member] | Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									50
Operating Income (Loss)									14
Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income (Loss)									1,106	1,031	958
Operating Segments [Member] | Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									139
Operating Income (Loss)									27
Operating Segments [Member] | ISR Systems [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income (Loss)									448	348	362
Operating Segments [Member] | ISR Systems [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,907	2,540	2,662
Operating Segments [Member] | ISR Systems [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									1,549	1,463	1,450
Operating Segments [Member] | C&NS [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income (Loss)									282	362	298
Operating Segments [Member] | C&NS [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,307	2,424	2,238
Operating Segments [Member] | C&NS [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									787	717	678
Operating Segments [Member] | Electronic Systems [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Income (Loss)									376	321	298
Operating Segments [Member] | Electronic Systems [Member] | Product [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									2,007	1,814	1,617
Operating Segments [Member] | Electronic Systems [Member] | Service [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales									$ 789	$ 695	$ 653