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

No definition available.

Name:	lm_DebtInstrumentBasisSpreadOnVariableRateBasisPointsHighEndOfRange
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.

+ References

No definition available.

+ Details

Name:	lm_DebtInstrumentBasisSpreadOnVariableRateBasisPointsLowEndOfRange
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
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.

+ References

No definition available.

+ Details

Name:	lm_GainOnDerivativeAssetsAndLiabilitiesRecordedInOtherNonOperatingIncomeOfConsolidatedInvestmentVehicles
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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

Reference 1: 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 2: 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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 3: 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=7491637&loc=d3e13433-108611

Reference 4: 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 5: 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 6: 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=7491637&loc=d3e13495-108611

+ Details

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

- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 30

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612

+ Details

Name:	us-gaap_FairValueOptionChangesInFairValueGainLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Variable Interest Entities and Consolidation of Investment Vehicles (Details 8) (CIVs, USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Estimate of Fair Value, Fair Value Disclosure	Mar. 31, 2012 Estimate of Fair Value, Fair Value Disclosure
Debt Instrument
Fair Value Option, Gain (Loss) recognized in other non-operating income of CIVs	$ (152)	$ (3,047)	$ (3,416)	$ (4,551)
Pre-defined interest rate spread of CLO debt, high end of range (as a percent)	4.00%		4.00%		4.00%		4.00%
Pre-defined interest rate spread of CLO debt, low end of range (as a percent)	0.25%		0.25%		0.25%		0.25%
Derivative liabilities									(3,514)	3,872
Gain on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles					106	41,896	358	53,986
Loss on Derivative Assets and Liabilities Recorded in Other Non-Operating Income of Consolidated Investment Vehicles					$ (305)	$ (35,025)	$ (604)	$ (46,836)