Income Taxes Schedule of Deferred Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	$ 158,369	$ 154,074
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Accrued Liabilities	60,282	61,575
Deferred Tax Assets, Operating Loss Carryforwards	290,765	267,940
Deferred Tax Assets, Capital Loss Carryforwards	5,335	10,015
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	247,027	235,661
Deferred Tax Assets, Other	3,817us-gaap_DeferredTaxAssetsOther	303us-gaap_DeferredTaxAssetsOther
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Pensions	7,741	0
Deferred Tax Assets, Gross	822,765us-gaap_DeferredTaxAssetsGross	778,256us-gaap_DeferredTaxAssetsGross
U.S. Earnings	3,500,000lm_U.S.Earnings
Deferred Tax Assets, Valuation Allowance	(96,687)	(90,832)
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Deferred Tax Liabilities, Goodwill and Intangible Assets	82,636	72,596
Deferred Tax Liabilities, Property, Plant and Equipment	666,057	523,595
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	7,832	4,743
Deferred Tax Liabilities, Other	78us-gaap_DeferredTaxLiabilitiesOther	221us-gaap_DeferredTaxLiabilitiesOther
Deferred Tax Liabilities, Deferred Expense	756,603	601,155
Deferred Tax Assets, Net	(30,525)us-gaap_DeferredTaxAssetsLiabilitiesNet	86,269us-gaap_DeferredTaxAssetsLiabilitiesNet
Federal Benefit of Uncertain Tax Positions [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Other	18,461	16,914
Fund Launch Costs [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Other	30,968	31,774
Domestic Tax Authority [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards	0	3,545
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		235,661
Deferred Tax Assets, Gross	702,233
State and Local Jurisdiction [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards	44	532
Deferred Tax Assets, Gross	186,944
Valuation Allowances and Reserves, Period Increase (Decrease)	34,600
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	9,359
Foreign Tax Authority [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Capital Loss Carryforwards	5,290	5,938
Valuation Allowances and Reserves, Period Increase (Decrease)	40,000
Martin Currie [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Valuation Allowance	(17,000)
Martin Currie [Member]
Schedule of Deferred Tax Assets [Line Items]
Deferred Tax Assets, Valuation Allowance	(18,441)
Valuation Allowance, Operating Loss Carryforwards [Member]
Schedule of Deferred Tax Assets [Line Items]
Income Tax Expense (Benefit)	$ 15,444

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