Derivatives and Hedging (Details) (Legg Mason, Inc., USD $) In Thousands	3 Months Ended	6 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Mar. 31, 2011
Derivative
Derivative Assets	$ 9,415		$ 9,415		$ 1,169
Derivative Liabilities	1,081		1,081		3,120
Gains recognized on derivative instruments not designated as hedging instruments	11,412	991	11,398	4,560
Losses recognized on derivative instruments not designated as hedging instruments	(3,430)	(5,966)	(4,250)	(6,084)
Forward foreign currency contracts
Derivative
Open currency forward contracts with aggregate gross asset fair values	8,417		8,417		1,112
Open currency forward contracts with aggregate gross liability fair values	1,056		1,056		1,633
Forward foreign currency contracts \| Operating activities - other expense
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	5,047	982	4,805	3,296
Losses recognized on derivative instruments not designated as hedging instruments	(2,835)	(3,244)	(3,456)	(3,956)
Forward foreign currency contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	401	0	299	6
Losses recognized on derivative instruments not designated as hedging instruments	(29)	(328)	(65)	(108)
Futures contracts
Derivative
Cash collateral	3,529		3,529		7,099
Open futures contracts with aggregate gross asset fair values	998		998		57
Open futures contracts with aggregate gross liability fair values	25		25		1,487
Futures contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	5,964	9	6,294	1,258
Losses recognized on derivative instruments not designated as hedging instruments	$ (566)	$ (2,394)	$ (729)	$ (2,020)

Derivative [Abstract]

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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Gain included in earnings for the period from the change in fair value of derivative instruments not designated as hedging instruments.

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Loss included in earnings for the period from the change in fair value of derivative instruments not designated as hedging instruments.

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Fair value as of the balance sheet date of foreign currency derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets on the balance sheet.

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Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities on the balance sheet.

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Fair value as of the balance sheet date of all price risk derivative assets not designated as hedging instruments.

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Fair value as of the balance sheet date of all price risk derivative liabilities not designated as hedging instruments.

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