Change in Assets Under Management

No definition available.

Name:	lm_ChangeinAssetsUnderManagement
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount used to represent the attrition when calculating the fair value of amortizable asset management contracts

+ References

No definition available.

+ Details

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

- Definition

Percentage of assumed long-term growth in AUM, used as an input to measure fair value.

+ References

No definition available.

+ Details

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

- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228

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=98514028&loc=d3e3602-108585

+ Details

Name:	us-gaap_AssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ 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 (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Definition

Amount after 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_FiniteLivedIntangibleAssetsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage of fair value of reporting unit in excess of carrying amount.

+ References

No definition available.

+ Details

Name:	us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=lm_AssetsManagementContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Intangible Assets and Goodwill Finite-Lived Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net			$ 164,928	$ 213,654
RARE Infrastructure, Ltd [Member] \| Customer Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Change in Assets Under Management	$ 1,500,000
Impairment of Intangible Assets	32,000	$ 18,000
Finite-Lived Intangible Assets, Net	$ 11,180
Fair Value Inputs, Long-term AUM Growth Rate		7.00%
Fair Value Inputs, Attrition Rate		(13.00%)
Fair Value Inputs, Discount Rate		15.50%
EnTrust Capital [Member] \| Customer Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Net			$ 51,854
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount			4.00%
Fair Value, Inputs, Level 3 [Member] \| Asset Management Contracts
Finite-Lived Intangible Assets [Line Items]
Fair Value Inputs, Long-term AUM Growth Rate	7.00%	7.00%
Fair Value Inputs, Attrition Rate	20.00%	13.00%
Fair Value Inputs, Discount Rate	16.50%	15.50%