Significant Accounting Policies (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
CIVs
Assets Issued by Collateralized Loan Obligations	$ 224,193	$ 291,853
Long-term debt of consolidated investment vehicles	207,835	271,707
Consolidated Legg Mason
Long-term debt of consolidated investment vehicles	207,835	271,707
Deferred Sales Commission	8,259	9,510
Gains (Losses) on Extinguishment of Debt	(68,975)	0	0
Cash and Cash Equivalents \| Consolidated Legg Mason
Cash and Cash Equivalents, Maturity Period	90
Debt \| Consolidated Legg Mason
Long-term Debt, Fair Value	1,206,166	1,214,245
Capital Plan \| Consolidated Legg Mason
Net reduction in outstanding debt	$ 350,000
other developments \| Consolidated Legg Mason
Profits interests equity plan participation percentage	15.00%
income tax policy \| Consolidated Legg Mason
Minimum Probability of Tax Benefit Recognition upon Settlement for Benefit to be Recorded	50.00%
investment advisory fees \| Consolidated Legg Mason
AUM eligible for performance fees	6.00%	6.00%	6.00%

Sum of the carrying values as of the balance sheet date of all assets issued by collateralized loan obligations.

No definition available.

Name:	lm_AssetsIssuedByCollateralizedLoanObligations
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of AUM eligible to earn performance fees.

+ References

No definition available.

+ Details

Name:	lm_AumEligibleForPerformanceFees
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the maximum original maturity of cash and cash equivalents, in days.

+ References

No definition available.

+ Details

Name:	lm_CashAndCashEquivalentsMaturityPeriod
Namespace Prefix:	lm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of probability of the tax benefit recognition upon settlement for a tax benefit to be recorded.

+ References

No definition available.

+ Details

Name:	lm_MinimumProbabilityOfTaxBenefitRecognitionUponSettlementForBenefitToBeRecorded
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Net reduction in outstanding debt.

+ References

No definition available.

+ Details

Name:	lm_NetReductionInOutstandingDebt
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sum of the carrying values as of the balance sheet date of all long-term debt of the consolidated investment vehicles, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year (current maturities) or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

+ References

No definition available.

+ Details

Name:	lm_NoncurrentLongTermDebtOfConsolidatedInvestmentVehicles
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the percentage of participation in future growth in value of the affiliate available for participation by key employees under the terms of a profits interest management equity plan.

+ References

No definition available.

+ Details

Name:	lm_ProfitsInterestsEquityPlanParticipationPercentage
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of advanced commission payments.

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredSalesCommission
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 26

-Paragraph 20, 21

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 50

-Section 40

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12317-112629

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 50

-Section 40

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12355-112629

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant