Contracts in Process (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Jan. 01, 2018
Contracts Receivable [Abstract]
Unbilled contract receivables, gross	$ 1,874		$ 2,716	$ 2,232
Unliquidated progress payments	(761)		(1,126)	(883)
Unbilled contract receivables, net	1,113
Inventoried contract costs, gross	891
Unliquidated progress payments	(71)
Inventoried contract costs, net	820
Contracts in process	1,933		$ 0	$ 0
Summary Of Inventory For Long Term Contracts Or Programs [Roll Forward]
Amounts included in inventoried contract costs at beginning of the year	173	$ 137
Contract costs incurred [Abstract]:
IRAD and B&P	333	304
Other G&A	842	749
Total	1,175	1,053
Amounts charged to cost of sales	(1,179)	(1,017)
Amounts included in inventoried contract costs at end of the year	$ 169	$ 173

Amounts charged to cost of sales.

No definition available.

Name:	lll_AmountsChargedToCostOfSales
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of costs incurred under contract costs during the period.

+ References

No definition available.

+ Details

Name:	lll_ContractCostsIncurred
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract Costs Incurred [Abstract]

+ References

No definition available.

+ Details

Name:	lll_ContractCostsIncurredAbstract
Namespace Prefix:	lll_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of general and administrative costs included in inventoried contract costs.

+ References

No definition available.

+ Details

Name:	lll_GeneralAndAdministrativeCostsIncludedInInventoriedContractCosts
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of general & administrative, independent research & development and bid & proposal costs included in inventoried contract costs.

+ References

No definition available.

+ Details

Name:	lll_GeneralAndAdministrativeIndependentResearchAndDevelopmentAndBidAndProposalCostsIncludedInInventoriedContractCosts
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Refers to the amount of Independent research and development and bid and proposal costs included in inventoried contract costs.

+ References

No definition available.

+ Details

Name:	lll_IndependentResearchAndDevelopmentAndBidAndProposalCostsIncludedInInventoriedContractCosts
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Summary Of Inventory For Long Term Contracts Or Programs [Roll Forward]

+ References

No definition available.

+ Details

Name:	lll_SummaryOfInventoryForLongTermContractsOrProgramsRollForward
Namespace Prefix:	lll_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before progress payments for unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be gross amounts associated with contracts or programs where the recognized revenue for performance there under exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

No definition available.

+ Details

Name:	lll_UnbilledContractReceivablesGross
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of advances received from customers under long-term contracts which have been offset against Unbilled Contract Receivables at the balance sheet date.

+ References

No definition available.

+ Details

Name:	lll_UnliquidatedProgressPaymentsForUnbilledContractReceivables
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer; classified as current.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount before valuation and LIFO reserves, and progress payments, of inventory associated with long-term contracts, expected to be sold, or consumed within one year or operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a),(d))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount, after deduction of valuation reserves, other allowances, and customer advances received on long-term contracts or programs, of inventory expected to be converted to cash, sold or exchanged within the normal operating cycle.

+ References

No definition available.

+ Details

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

- Definition

Amount of advances received from customers under long-term contracts which have been offset against inventories accounts at the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(d)(iii))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 912
-SubTopic 310
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=68054760&loc=d3e55302-109406

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(c)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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