The basis points added to the reference rate to compute the variable rate on the debt instrument.

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Namespace Prefix:	lm
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The basis points added to the reference rate to compute the variable rate on the debt instrument.

+ Details

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Namespace Prefix:	lm
Data Type:	num:percentItemType
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Period Type:	instant

- Definition

This element represents the realized and unrealized gain on derivative assets and liabilities recorded in other nonoperating income of consolidated investment vehicles.

+ Details

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Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the realized and unrealized loss on derivative assets and liabilities recorded in other nonoperating income of consolidated investment vehicles.

+ Details

Name:	lm_LossOnDerivativeAssetsAndLiabilitiesRecordedInOtherNonOperatingIncomeOfConsolidatedInvestmentVehicles
Namespace Prefix:	lm
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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13495-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FIN39-1

-Paragraph 10A, 10B

-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.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 4, 17

-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.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611

+ Details

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Period Type:	instant

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References

+ Details

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

Variable Interest Entities and Consolidation of Investment Vehicles (Details 7) (USD $) In Thousands, unless otherwise specified				3 Months Ended		6 Months Ended
	Sep. 30, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] CIVs	Mar. 31, 2011 Estimate of Fair Value, Fair Value Disclosure [Member] CIVs	Sep. 30, 2011 CLO-VIE	Sep. 30, 2011 CIVs	Sep. 30, 2010 CIVs	Sep. 30, 2011 CIVs	Sep. 30, 2010 CIVs
Debt Instrument [Line Items]
Pre-defined interest rate spread of CLO debt, high end of range (as a percent)			4.00%
Pre-defined interest rate spread of CLO debt, low end of range (as a percent)			0.25%
Derivative assets		$ 170
Derivative Liabilities	(4,285)	(14,297)
Gain on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles				41,896	4,811	53,986	6,174
Loss on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles				$ (35,025)	$ (5,387)	$ (46,836)	$ (7,926)