Stock-Based Compensation (Details 4) (Equity Plan for Non-employee Director [Member], USD $) In Thousands	6 Months Ended
Sep. 30, 2011	Sep. 30, 2010
Restricted stock unit
Stock based compensation disclosure
Stock and units granted (in shares)	12	17
Restricted stock units outstanding (in shares)	74	62
Stock Distributed During Period, Shares Restricted Stock Award under Non-Employee Director Plan		7
Stock Option [Member]
Stock based compensation disclosure
Stock options outstanding (in shares)	193	220
Stock options exercised (in shares)		9
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period	27	59
Common Stock [Member]
Stock based compensation disclosure
Stock and units granted (in shares)	31	31
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value	1,375	1,425

This element represents the number of restricted stock units that are outstanding under the non-employee director plan.

Name:	lm_RestrictedStockUnitsOutstandingToNonEmployeeDirectorNumberOfShares
Namespace Prefix:	lm
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares distributed during the period, as a result of the restricted stock awards, under the non-employee director plan.

+ Details

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Namespace Prefix:	lm
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

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+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The total fair value of equity-based awards for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

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+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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.

+ Details

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Period Type:	duration

- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(1)(f)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Name Accounting Standards Codification

-Topic 718

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+ Details

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- Definition

The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

+ References

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-Publisher FASB

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-Paragraph 2

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Number of share options (or share units) exercised during the current period.

+ References

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-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

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+ Details

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Balance Type:	na
Period Type:	duration