Derivatives and Hedging (Details) (Legg Mason, Inc., USD $) In Thousands, unless otherwise specified	3 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2012
Forward foreign currency contracts
Derivative
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Asset at Fair Value	$ 1,125		$ 38
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	276		685
Forward foreign currency contracts \| Operating activities - other expense
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	1,400	1,986
Losses recognized on derivative instruments not designated as hedging instruments	(373)	(2,860)
Forward foreign currency contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	287	30
Losses recognized on derivative instruments not designated as hedging instruments	(30)	(158)
Futures contracts
Derivative
Cash collateral	2,327		1,919
Open futures contracts with aggregate gross asset fair values	4		46
Open futures contracts with aggregate gross liability fair values	486		201
Futures contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	1,014	1,661
Losses recognized on derivative instruments not designated as hedging instruments	(541)	(1,494)
Derivatives - Total
Derivative
Derivative Assets	1,129		84
Derivative Liabilities	762		886
Gains recognized on derivative instruments not designated as hedging instruments	2,701	3,677
Losses recognized on derivative instruments not designated as hedging instruments	$ (944)	$ (4,512)

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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 all foreign currency derivative assets not designated as hedging instruments.

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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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