Amounts charged to cost of sales.

No definition available.

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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

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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

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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

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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

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Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- 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
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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

- References

No definition available.

+ Details

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

- Definition

Amount of receivable reflecting the cost incurred on uncompleted contracts in excess of related billings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(c)(4))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3)(c)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 35
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68068069&loc=d3e57795-111642

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 35
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68068069&loc=d3e57807-111642

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

+ Details

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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

+ Details

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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://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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

Contracts in Process (Details) - USD ($) $ in Millions	3 Months Ended
Contracts in Process (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Contractors [Abstract]
Unbilled contract receivables, gross		$ 2,481	$ 2,232	$ 1,874
Unliquidated progress payments		(982)	(883)	(761)
Unbilled contract receivables, net				1,113
Inventoried contract costs, gross				891
Unliquidated progress payments				(71)
Inventoried contract costs, net				820
Total contracts in process		$ 0	$ 0	$ 1,933
Summary Of Inventory For Long Term Contracts Or Programs [Roll Forward]
Amounts included in inventoried contract costs at beginning of the period	$ 173
Contract Costs Incurred [Abstract]
IRAD and B&P	76
Other G&A	200
Total	276
Amounts charged to operating costs and expenses	(272)
Amounts included in inventoried contract costs at end of the period	$ 177