Derivatives and Hedging (Details) (Legg Mason, Inc., USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011
Derivative
Derivative Assets	$ 10,204		$ 10,204		$ 1,169
Derivative Liabilities	1,402		1,402		3,120
Gains recognized on derivative instruments not designated as hedging instruments	4,096	1,433	12,985	5,805
Losses recognized on derivative instruments not designated as hedging instruments	(3,385)	(4,003)	(5,128)	(9,904)
Forward foreign currency contracts
Derivative [Line Items]
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	853		853		1,633
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Asset at Fair Value	10,169		10,169		1,112
Derivative
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Asset at Fair Value	10,169		10,169		1,112
Foreign Currency Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value	853		853		1,633
Forward foreign currency contracts \| Operating activities - other expense
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	3,628	1,184	6,869	4,356
Losses recognized on derivative instruments not designated as hedging instruments	(946)	(1,083)	(2,839)	(4,917)
Forward foreign currency contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	270	85	447	71
Losses recognized on derivative instruments not designated as hedging instruments	(149)	0	(93)	(88)
Futures contracts
Derivative
Cash collateral	4,240		4,240		7,099
Open futures contracts with aggregate gross asset fair values	35		35		57
Open futures contracts with aggregate gross liability fair values	549		549		1,487
Futures contracts \| Seed Capital Investments - Other non-operating income (expense)
Derivative
Gains recognized on derivative instruments not designated as hedging instruments	198	164	5,669	1,378
Losses recognized on derivative instruments not designated as hedging instruments	$ (2,290)	$ (2,920)	$ (2,196)	$ (4,899)

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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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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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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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