Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: 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

+ Details

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

- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAverageRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeFixedInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

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

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_FutureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_DerivativeFinancialInstrumentsAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	invest_InvestmentHoldingAxis=lm_LeggMasonGlobalInfrastructureETFMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	invest_InvestmentHoldingAxis=lm_ClearBridgeAllCapGrowthETFMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivatives and Hedging Derivative Instruments and Hedging Activities Disclosure (Details) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2017	Jul. 26, 2017	Jun. 06, 2017	Mar. 31, 2017	Jun. 30, 2016
Derivative
Derivative, Term of Contract	4 years 245 days
Derivative, Notional Amount		$ 23,096	$ 20,253
Other Assets [Member]
Derivative
Amount Net for Fair Value of Derivative Assets and (Liabilities)	$ 5,109			$ 2,718
Other Liabilities [Member]
Derivative
Amount Net for Fair Value of Derivative Assets and (Liabilities)	3,167			$ 4,522
Foreign Exchange Forward [Member]
Derivative
Derivative, Notional Amount	$ 221,009
Derivative, Average Remaining Maturity	6 months
Future [Member]
Derivative
Derivative, Notional Amount	$ 147,988
Derivative, Average Remaining Maturity	3 months
Derivative Financial Instruments, Assets [Member] \| Future [Member]
Derivative
Derivative, Notional Amount	$ 44,464
Revolving Credit Facility [Member] \| Swap [Member]
Derivative
Derivative, Term of Contract	4 years 245 days
Derivative, Notional Amount					$ 500,000
Derivative, Fixed Interest Rate					2.30%
Legg Mason Global Infrastructure ETF [Member]
Derivative
Derivative, Notional Amount	$ 23,588	$ 23,096
Legg Mason Global Infrastructure ETF [Member] \| Swap [Member]
Derivative
Derivative, Fixed Interest Rate	1.60%
Legg Mason Global Infrastructure ETF [Member] \| Derivative Financial Instruments, Assets [Member] \| Future [Member]
Derivative
Derivative, Notional Amount	$ 40,090
ClearBridge All Cap Growth ETF [Member]
Derivative
Derivative, Notional Amount			$ 20,253
ClearBridge All Cap Growth ETF [Member] \| Swap [Member]
Derivative
Derivative, Notional Amount	$ 20,876
Derivative, Fixed Interest Rate	1.35%
ClearBridge All Cap Growth ETF [Member] \| Derivative Financial Instruments, Assets [Member] \| Future [Member]
Derivative
Derivative, Notional Amount	$ 20,511