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=26713463&loc=d3e16265-109275

+ Details

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

- Definition

The interest rate used to adjust for the time value of money.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(1)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

+ Details

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

- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Expected Long-Term Rate of Return on Plan Assets

-URI http://asc.fasb.org/extlink&oid=6512046

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

+ Details

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

- Definition

Weighted average expected volatility of stock price.

+ 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 (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsWeightedAverageVolatilityRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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=36462937&loc=d3e19207-110258

+ Details

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

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_MartinCurrieMember
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:

- Details

Name:	lm_FairValueInputsMarketDataInputsVariablesbyTypeAxis=lm_AssetsUnderManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lm_FairValueInputsMarketDataInputsVariablesbyTypeAxis=lm_PerformanceFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	lm_FairValueInputsMarketDataInputsVariablesbyTypeAxis=lm_EarnoutPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Acquisitions Martin Currie Acquisition - Fair Value Assumptions (Details) - Scenario, Unspecified [Domain] - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
Fund management contracts [Member] \| Martin Currie [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			12 years
Fund management contracts [Member] \| Martin Currie [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Input, Projected Cash Flow Growth Rate, Low End of Range			0.00%
Fair Value Input, Projected Cash Flow Growth Rate, High End of Range			25.00%
Fair Value Input, Projected Cash Flow Growth Rate, Average			11.00%
Fair Value Inputs, Discount Rate			15.00%
Asset Management Contracts \| Martin Currie [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate			15.00%
Fair Value Inputs, Long-term AUM Growth Rate			6.00%
Fair Value Inputs, Attrition Rate			(17.00%)
Assets Under Management [Member] \| Martin Currie [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Input, Projected Cash Flow Growth Rate, Low End of Range			0.00%
Fair Value Input, Projected Cash Flow Growth Rate, High End of Range			28.00%
Fair Value Input, Projected Cash Flow Growth Rate, Average			14.00%
Fair Value Inputs, Discount Rate			13.00%
Fair Value Assumptions, Weighted Average Volatility Rate			18.80%
Performance Fees [Member] \| Martin Currie [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Input, Projected Cash Flow Growth Rate, Low End of Range			0.00%
Fair Value Input, Projected Cash Flow Growth Rate, High End of Range			30.00%
Fair Value Input, Projected Cash Flow Growth Rate, Average			15.00%
Fair Value Inputs, Discount Rate			15.00%
Earn-out Payments [Member] \| Martin Currie [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate			1.30%
Martin Currie [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.80%	3.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	5.20%	6.30%
Revenues	$ 13,937