VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES (Details 5) (CIVs, USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2011
Nature of Investments
Fair value determined using NAV	$ 66,238
Unfunded Commitments	11,830
Hedge funds
Nature of Investments
Fair value determined using NAV	45,978
Percentage of investment subject to quarterly redemption (as a percent)	30.00%
Percentage of investment subject to annual redemption (as a percent)	1.00%
Percentage of quarterly investment subject to lock-up or side pocket provisions (as a percent)	69.00%
Investment lock up period or side pocket provisions period, low end of range (in years)	3
Investment lock up period or side pocket provisions period, high end of range (in years)	5
Funds-of-hedge funds
Nature of Investments
Fair value determined using NAV	2,381
Private equity funds
Nature of Investments
Fair value determined using NAV	17,879
Unfunded Commitments	$ 11,830
Remaining term (in years)	8

- Definition

For investments that do not have readily determinable fair values, this element represents the fair value of investments determined using net asset value.

+ Details

Name:	lm_FairValueDeterminedUsingNAV
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The period of time that the investment is subject to a lock-up, high end of range, in years.

+ Details

Name:	lm_InvestmentLockUpPeriodHighEndOfRange
Namespace Prefix:	lm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The period of time that the investment is subject to a lock-up, low end of range, in years.

+ Details

Name:	lm_InvestmentLockUpPeriodLowEndOfRange
Namespace Prefix:	lm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

This element represents the percentage of the investment subject to annual redemption.

+ Details

Name:	lm_PercentageOfAnnualRedemption
Namespace Prefix:	lm
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the percentage of the investment subject to quarterly redemption.

+ Details

Name:	lm_PercentageOfQuarterlyRedemption
Namespace Prefix:	lm
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the percentage of the investment subject to lock-up or side pocket provisions.

+ Details

Name:	lm_PercentageSubjectToLockInPeriod
Namespace Prefix:	lm
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The remaining term of the investment, in years.

+ Details

Name:	lm_RemainingTermOfInvestments
Namespace Prefix:	lm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

For investments that do not have readily determinable fair values, this element represents the unfunded commitments which may impact the ultimate value realized from the investment.

+ Details

Name:	lm_UnfundedCommitments
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration