Intangible Assets and Goodwill (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Mar. 31, 2011
Intangible assets
Amount domestic funds fair value exceeds carrying value	5.00%
Consolidated Legg Mason Group [Member]
Intangible assets
Indefinite-life intangible assets	$ 3,823,228	$ 3,823,457
Intangible assets, net	3,860,587	3,876,775
Consolidated Legg Mason Group [Member] \| Amortizable asset management contracts
Intangible assets
Cost	206,928	208,454
Accumulated amortization	(169,569)	(155,136)
Amortizable asset management contracts, net	37,359	53,318
Consolidated Legg Mason Group [Member] \| Fund management contracts
Intangible assets
Indefinite-life intangible assets	3,753,428	3,753,657
Consolidated Legg Mason Group [Member] \| Trade names
Intangible assets
Indefinite-life intangible assets	69,800	69,800
Consolidated Legg Mason Group [Member] \| Domestic Mutual Fund Contracts [Domain]
Intangible assets
Indefinite-life intangible assets	$ 2,502,000

The amount that the Legg Mason domestic mutual fund contracts' fair value exceeds their carrying value.

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The accumulated amount of amortization of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

+ References

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- Definition

Sum of the gross carrying amounts before accumulated amortization as of the balance sheet date of all intangible assets having statutory or estimated useful lives. The aggregate gross carrying amount (including any previously recognized impairment charges) of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

+ References

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The aggregate sum of gross carrying value of a major finite-lived intangible asset class, less accumulated amortization and any impairment charges. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

+ References

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Sum of the carrying amounts (original costs for current and prior period additions adjusted for impairment, if any) as of the balance sheet date of intangible assets, excluding goodwill, having a projected indefinite period of benefit.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-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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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

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-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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