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Stock-Based Compensation - Additional Information (Detail) (USD $)
3 Months Ended
Mar. 27, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Exercise price $ 129.31us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Stock options granted 444,454us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Expiration period of stock options granted from date of grant 10 years
Vesting period 3 years
Weighted average grant date fair value of stock options $ 19.82us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Restricted Stock Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period 3 years
Units granted 350,063us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
Weighted average grant date fair value $ 129.31us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
Restricted Stock Units [Member] | Non-Employee Directors [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Vesting period 1 year
Performance Units [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Units granted 40,252us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= lll_PerformanceUnitsMember
Weighted average grant date fair value $ 129.31us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= lll_PerformanceUnitsMember
Length of service after grant date 3 years
Minimum payout range 0.00%lll_MinimumPayoutRange
/ us-gaap_AwardTypeAxis
= lll_PerformanceUnitsMember
Maximum payout range 200.00%lll_MaximumPayoutRange
/ us-gaap_AwardTypeAxis
= lll_PerformanceUnitsMember