VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES (Details 8) (USD $) In Thousands	Mar. 31, 2011	Mar. 31, 2010
Variable Interest Entity Disclosures
VIE Assets Not Consolidated	$ 21,316,932	$ 20,484,006
VIE Liabilities Not Consolidated	373,465	3,217,224
Equity Interests on the Consolidated Balance Sheet	83,770	103,010
Maximum Risk of Loss	122,385	143,628
CDOs/CLOs
Variable Interest Entity Disclosures
VIE Assets Not Consolidated	382,692	3,508,290
VIE Liabilities Not Consolidated	354,692	3,215,890
Maximum Risk of Loss	196
VIEs for which Legg Mason no longer has a variable interest, total assets		2,817,357
VIEs for which Legg Mason no longer has a variable interest, total liabilities		2,577,457
Public-Private Investment Program
Variable Interest Entity Disclosures
VIE Assets Not Consolidated	692,488	411,489
VIE Liabilities Not Consolidated	2,002
Equity Interests on the Consolidated Balance Sheet	290	55,526
Maximum Risk of Loss	290	72,245
Other sponsored investment funds
Variable Interest Entity Disclosures
VIE Assets Not Consolidated	20,241,752	16,564,227
VIE Liabilities Not Consolidated	16,771	1,334
Equity Interests on the Consolidated Balance Sheet	83,480	47,484
Maximum Risk of Loss	$ 121,899	$ 71,383

- Definition

Represents the carrying amount of the assets in the enterprise's statement of operations that relate to the enterprise's variable interest in the unconsolidated variable interest entity.

+ Details

Name:	lm_VariableInterestEntityCarryingAmountOfCollateralizedAssetsUnconsolidated
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the carrying amount of the liabilities in the enterprise's statement of financial position that relate to the enterprise's variable interest in the unconsolidated variable interest entity.

+ Details

Name:	lm_VariableInterestEntityCarryingAmountOfCollateralizedLiabilitiesUnconsolidated
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the carrying amount of the equity in the enterprise's Statement of Financial Position that relates to the enterprise's variable interest in the unconsolidated variable interest entity.

+ Details

Name:	lm_VariableInterestEntityEquityInterestInConsolidatedBalanceSheet
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the assets of Variable Interest Entities in which Legg Mason is no longer considered to have a variable interest.

+ Details

Name:	lm_VariableInterestEntityLeggMasonNoLongerHasVariableInterestCarryingAmountAsset
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of the liabilities of Variable Interest Entities in which Legg Mason is no longer considered to have a variable interest.

+ Details

Name:	lm_VariableInterestEntityLeggMasonNoLongerHasVariableInterestCarryingAmountLiability
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents enterprise's maximum exposure to loss as a result of its involvement with the VIE.

+ Details

Name:	lm_VariableInterestEntityMaximumRiskOfLoss
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration