L-3 Holdings' Earnings Per Common Share - Reconciliation of Basic and Diluted Earnings per Share (EPS) (Parenthetical) (Detail) - CODES Due 2035 [Member] - $ / shares	6 Months Ended
Jun. 26, 2015	Jun. 27, 2014	Jun. 18, 2014
Subsidiary, Sale of Stock [Line Items]
Interest rate on debt instruments		3.00%
Debt instruments, maturity year	2035
Conversion price of L-3 Holdings' CODES			$ 88.71
Debt instruments, retired date	Jun. 20, 2014

Debt Instrument Maturity Year

No definition available.

Name:	lll_DebtInstrumentMaturityYear
Namespace Prefix:	lll_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument retired date.

+ References

No definition available.

+ Details

Name:	lll_DebtInstrumentRetiredDate
Namespace Prefix:	lll_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870

+ Details

Name:	us-gaap_DebtInstrumentConvertibleConversionPrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Period Type: