Percentage of assumed growth rate in projected cash flows, used as an input to measure fair value, average

No definition available.

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

- Definition

Percentage of assumed growth rate in projected cash flows, used as an input to measure fair value, high end of range

+ References

No definition available.

+ Details

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

- Definition

Percentage of assumed growth rate in projected cash flows, used as an input to measure fair value, low end of range

+ References

No definition available.

+ Details

Name:	lm_FairValueInputProjectedCashFlowGrowthRateLowEndOfRange
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
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

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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_FairValueInputsAssetsQuantitativeInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=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=us-gaap_TradeNamesMember
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:

Acquisitions EnTrust Capital Fair Value Inputs (Details)	3 Months Ended		12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Mar. 31, 2016
Fund management contracts \| EnTrust Capital [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			8 years
Fund management contracts \| EnTrust Capital [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input, Projected Cash Flow Growth Rate, Low End of Range		(1.00%)
Fair Value Input, Projected Cash Flow Growth Rate, High End of Range		5.00%
Fair Value Input, Projected Cash Flow Growth Rate, Average		4.00%
Fair Value Inputs, Discount Rate		14.50%
Trade Names \| EnTrust Capital [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input, Projected Cash Flow Growth Rate, Low End of Range		6.00%
Fair Value Input, Projected Cash Flow Growth Rate, High End of Range		14.00%
Fair Value Input, Projected Cash Flow Growth Rate, Average		9.00%
Fair Value Inputs, Discount Rate		14.50%
Asset Management Contracts \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	16.50%	15.50%
Fair Value Inputs, Long-term AUM Growth Rate	7.00%	7.00%
Fair Value Inputs, Attrition Rate	(20.00%)	(13.00%)
Asset Management Contracts \| EnTrust Capital [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate			13.50%
Fair Value Inputs, Long-term AUM Growth Rate			10.00%
Fair Value Inputs, Attrition Rate			(13.00%)