Accounting Standards Issued and Not Yet Implemented - Additional Information (Detail)	9 Months Ended	12 Months Ended
Sep. 23, 2016	Dec. 31, 2015
Accounting Changes and Error Corrections [Abstract]
Percentage of sales accounted for under contract accounting standards	50.00%	49.00%
Percentage of net sales accounted based on units-of-delivery method	15.00%

Percentage of sales accounted for under contract accounting standards based on the units-of-delivery method.

No definition available.

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