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Acquisitions Martin Currie Acquisition - Fair Value Assumptions (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended 6 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2013
Mar. 31, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]        
Revenues $ 2,819,106us-gaap_Revenues $ 2,741,757us-gaap_Revenues $ 2,612,650us-gaap_Revenues  
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 0 days      
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%lm_FairValueInputProjectedCashFlowGrowthRateLowEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, High End of Range 25.00%lm_FairValueInputProjectedCashFlowGrowthRateHighEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, Average 11.00%lm_FairValueInputProjectedCashFlowGrowthRateAverage
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Fair Value Inputs, Discount Rate 15.00%us-gaap_FairValueInputsDiscountRate
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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%us-gaap_FairValueInputsDiscountRate
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Fair Value Inputs, Long-term AUM Growth Rate 6.00%lm_FairValueInputsLongtermAUMGrowthRate
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Fair Value Inputs, Attrition Rate (17.00%)lm_FairValueInputsAttritionRate
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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%lm_FairValueInputProjectedCashFlowGrowthRateLowEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, High End of Range 28.00%lm_FairValueInputProjectedCashFlowGrowthRateHighEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, Average 14.00%lm_FairValueInputProjectedCashFlowGrowthRateAverage
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Fair Value Inputs, Discount Rate 13.00%us-gaap_FairValueInputsDiscountRate
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Fair Value Assumptions, Weighted Average Volatility Rate 18.80%us-gaap_FairValueAssumptionsWeightedAverageVolatilityRate
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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%lm_FairValueInputProjectedCashFlowGrowthRateLowEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, High End of Range 30.00%lm_FairValueInputProjectedCashFlowGrowthRateHighEndOfRange
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Fair Value Input, Projected Cash Flow Growth Rate, Average 15.00%lm_FairValueInputProjectedCashFlowGrowthRateAverage
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Fair Value Inputs, Discount Rate 15.00%us-gaap_FairValueInputsDiscountRate
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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%us-gaap_FairValueInputsDiscountRate
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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 3.30%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
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Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 6.30%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
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Revenues       $ 32,293us-gaap_Revenues
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