Gain (loss) on sale of business net of tax.

No definition available.

Name:	lll_GainLossOnSaleOfBusinessNetOfTax
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Date when the business acquisition agreement was executed, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount classified as defined benefit plan benefit obligations attributable to disposal group held for sale or disposed of.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationPensionPlanBenefitObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Gain (loss) from sale or disposal of an organization or integrated set of activities (for example, but not limited to, a partnership or corporation) engaged in providing a product or service in a commercial, industrial, or professional environment.

+ References

No definition available.

+ Details

Name:	us-gaap_GainLossOnSaleOfBusiness
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=lll_CACIInternationalIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=lll_MarineSystemsInternationalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=lll_BroadcastSportsInternationalLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=lll_TinsleyProductLineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_CTCAviationGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_MiteqIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_AdvancedTechnicalMaterialsIncorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_ForceXIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_ImplantSciencesCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lll_MicreoLimitedAndAerosimTechnologiesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Divestitures and Acquisitions - Additional Information (Detail) € in Millions, £ in Millions		3 Months Ended		9 Months Ended
	Feb. 01, 2016 USD ($)	Sep. 23, 2016 USD ($)	Sep. 25, 2015 USD ($)	Sep. 23, 2016 USD ($)	Sep. 25, 2015 USD ($)	Oct. 10, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jan. 22, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 13, 2015 USD ($)	Jul. 27, 2015 USD ($)	May 29, 2015 USD ($)	May 29, 2015 EUR (€)	May 27, 2015 USD ($)	May 27, 2015 GBP (£)	Apr. 24, 2015 USD ($)	Jan. 21, 2015 USD ($)
Business Acquisition [Line Items]
Gain (loss) related to business divestiture			$ (9,000,000)		$ (29,000,000)
Amount of goodwill recognized		$ 6,284,000,000		$ 6,284,000,000					$ 6,281,000,000
CACI International Inc [Member]
Business Acquisition [Line Items]
Proceeds from sale of discontinued operations	$ 547,000,000
MSI [Member]
Business Acquisition [Line Items]
Sale of business, base sales price												$ 318,000,000	€ 295
Estimated employee pension related liabilities \| €													€ 60
Gain (loss) related to business divestiture					(17,000,000)
Gain (loss) related to business divestiture after income taxes					(6,000,000)
Broadcast Sports International, LLC [Member]
Business Acquisition [Line Items]
Sale of business, base sales price																$ 26,000,000
Gain (loss) related to business divestiture			(1,000,000)		(4,000,000)
Gain (loss) related to business divestiture after income taxes					(6,000,000)
Broadcast Sports International, LLC [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Gain (loss) related to business divestiture after income taxes			(1,000,000)
Tinsley Product Line [Member]
Business Acquisition [Line Items]
Sale of business, base sales price											$ 4,000,000
Gain (loss) related to business divestiture			(8,000,000)		(8,000,000)
Gain (loss) related to business divestiture after income taxes			$ (6,000,000)		$ (6,000,000)
CTC Aviation Group [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition														$ 236,000,000	£ 153
Acquisition date				May 27, 2015
Amount of goodwill recognized														$ 182,000,000
MITEQ, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition																	$ 41,000,000
Acquisition date				Jan. 21, 2015
Amount of goodwill recognized																	11,000,000
Goodwill deductible for income tax purposes																	$ 4,000,000
Advanced Technical Materials, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition								$ 27,000,000
Acquisition date				Jan. 22, 2016
Amount of goodwill recognized								$ 20,000,000
ForceX, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition										$ 61,000,000
Acquisition date				Oct. 13, 2015
Amount of goodwill recognized										53,000,000
Goodwill deductible for income tax purposes										$ 52,000,000
Implant Sciences Corporation [Member]
Business Acquisition [Line Items]
Acquisition date				Oct. 10, 2016
Implant Sciences Corporation [Member] \| Subsequent Event [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition						$ 118,000,000
Micreo Limited and Aerosim Technologies Inc [Member]
Business Acquisition [Line Items]
Acquisition date				Sep. 30, 2016
Micreo Limited and Aerosim Technologies Inc [Member] \| Subsequent Event [Member]
Business Acquisition [Line Items]
Aggregate purchase price of acquisition							$ 82,000,000