Basis of Presentation - Summary of Net Sales by Revenue Recognition Method as Percentage of Total Net Sales (Details)	3 Months Ended
Mar. 30, 2018
Revenue from Contract with Customer [Abstract]
Cost to cost method	76.00%
Point in time	18.00%
Output method	3.00%
Billing method	3.00%
Total	100.00%

Sales from Performance Obligations for Standard Services Measured over the Life of the Service as Percent of Net Sales

No definition available.

Name:	lll_SalesfromPerformanceObligationsforStandardServicesMeasuredovertheLifeoftheServiceasPercentofNetSales
Namespace Prefix:	lll_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration