Components Intangible Assets (Details 1) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Finite-Lived and Indefinite-lived Intangible Assets
Cost	$ 374,938	$ 408,025
Accumulated amortization	(199,315)	(194,371)
Total	175,623	213,654
Indefinite-life intangible assets	3,823,903	3,820,726
U.S. Domestic Mutual Fund Management Contracts
Finite-Lived and Indefinite-lived Intangible Assets
Indefinite-life intangible assets	2,106,351	2,106,351
Other Fund Management Contracts [Member]
Finite-Lived and Indefinite-lived Intangible Assets
Indefinite-life intangible assets	547,917	542,908
Trade Names
Finite-Lived and Indefinite-lived Intangible Assets
Indefinite-life intangible assets	68,031	69,863
EnTrustPermal [Member] \| Other Fund Management Contracts [Member]
Finite-Lived and Indefinite-lived Intangible Assets
Indefinite-life intangible assets	596,404	596,404
Clarion Partners [Member] \| Other Fund Management Contracts [Member]
Finite-Lived and Indefinite-lived Intangible Assets
Indefinite-life intangible assets	505,200	505,200
Consolidated Legg Mason, Inc.
Finite-Lived and Indefinite-lived Intangible Assets
Intangible assets, net	$ 3,999,526	$ 4,034,380

Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]

No definition available.

Name:	lm_FiniteLivedAndIndefiniteLivedIntangibleAssetsLineItems
Namespace Prefix:	lm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

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

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

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

- Definition

Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph ((a)(1),(b))
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=lm_USDomesticMutualFundManagementContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=lm_OtherFundManagementContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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