Fair Values of Assets and Liabilities (Details 2) (Legg Mason, Inc., USD $) In Thousands, unless otherwise specified	3 Months Ended
Jun. 30, 2012	Jun. 30, 2011
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	$ 207,103	$ 199,925
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	90	1,330
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(1,602)	(13,298)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(5,765)	(12,293)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net	713	3,169
Assets measured at fair value using significant unobservable inputs, value at end of period	200,539	178,833
Unrealized gains (losses) on Level 3 assets and liabilities still held at the reporting date	1,284	1,068
Trading proprietary fund products and other investments
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period		11,378
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(11,641)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0
Realized and unrealized gains/(losses), net		543
Assets measured at fair value using significant unobservable inputs, value at end of period		280
Equity method investments in proprietary fund products
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	11,778	12,167
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net	(482)	73
Assets measured at fair value using significant unobservable inputs, value at end of period	11,296	12,240
Investments in partnerships, LLCs and other
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	28,763	22,167
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(151)	(215)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net	(99)	0
Assets measured at fair value using significant unobservable inputs, value at end of period	28,513	21,952
Equity method investments in partnerships and LLCs
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	166,438	153,931
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	90	1,330
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(1,602)	(1,657)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(5,614)	(12,078)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net	1,290	1,581
Assets measured at fair value using significant unobservable inputs, value at end of period	160,602	143,107
Other Investments
Fair Value Assets and Liabilities Measured on Recurring Basis Financial Statement Unobservable Input Level 3
Assets measured at fair value using significant unobservable inputs, value at beginning of period	124	282
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0
Realized and unrealized gains/(losses), net	4	972
Assets measured at fair value using significant unobservable inputs, value at end of period	$ 128	$ 1,254

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This element represents total gains or losses for the period (unrealized), arising from assets and liabilities still held at the reporting date measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

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- Definition

Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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- Definition

Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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- Definition

Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

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- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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