Number of funds that are seeded by employee's in which Legg Mason is the primary beneficiary.

No definition available.

Name:	lm_EmployeeOwnedFunds
Namespace Prefix:	lm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of sponsored investment fund variable interest entities in which the reporting entity is the primary beneficiary.

+ References

No definition available.

+ Details

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

- Definition

The number of collateralized investment vehicles considered to be variable interest entities of the reporting entity.

+ References

No definition available.

+ Details

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

- Definition

The number of collateralized investment vehicles that are considered to be variable interest entities and for which the reporting entity is the primary beneficiary.

+ References

No definition available.

+ Details

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

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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.

+ References

No definition available.

+ Details

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

- Details

Name:	dei_LegalEntityAxis=lm_ConsolidatedInvestmentVehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=lm_ProprietaryFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Variable Interest Entities and Consolidation of Investment Vehicles Information about Investments in CIV's (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015
Variable Interest Entity
Employee Owned Funds		1	14	17
Variable Interest Entity, Number of Collateralized Securites Vehicles			3
Variable Interest Entity, Primary Beneficiary, Number of Collateralized Securites Vehicles		0	0
Number of Sponsored Investment Fund Vie	11	5	1
Consolidated Investment Vehicles [Member]
Variable Interest Entity
Assets, Fair Value Disclosure		$ 58,725
Consolidated Investment Vehicles [Member] \| Proprietary Funds [Member]
Variable Interest Entity
Assets, Fair Value Disclosure		$ 28,300	$ 13,641