LIQUIDITY FUND SUPPORT (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2010	Mar. 31, 2009
Liquidity fund support:
Pre Tax Gain (Charge)	$ 23,171	$ (2,283,236)
Cash paid for non-bank sponsored SIV securities purchase		2,868,815
Realized loss related to selling unsupported SIV securities		(2,257,217)
Non-asset Backed Securities
Liquidity fund support:
Pre Tax Gain (Charge)	23,171
After Tax Gain (Charge)	16,565
Gains on foreign exchange forward contract	1,484
Interest payments received related to SIV securities	1,056
Cash paid for non-bank sponsored SIV securities purchase		2,923,666
Accrued interest on non-bank sponsored SIV securities purchased		24,256
Principal amount of non-bank sponsored SIV securities purchased		2,972,772
Number of liquidity funds previously supported under CSAs and LOCs from which SIV securities were purchased		6
Securities sold that were previously supported by a total return swap		354,934
Canadian conduit securities sold		76,237
Proceeds from sale of previously supported securities and Canadian conduit securities, excluding transaction costs		654,726
Realized loss related to selling unsupported SIV securities		2,261,365
Realized loss related to selling unsupported SIV securities, net of taxes and operating expense adjustments		1,362,146
Loss related to reimbursements to two funds for a portion of losses incurred in selling SIV securities		181,183
Number of liquidity funds reimbursed for losses		2
Number of capital support agreements entered into during the period		4
Purchased Structured Investment Vehicles and Supported Non-asset Backed Securities
Liquidity fund support:
Unrealized losses related to SIV securities and supported non-asset backed securities		21,871
Unrealized losses related to SIV securities and supported non-asset backed securities, net of taxes and operating expense adjustments		$ 14,433

Represents the amount of conduit securities issued by a foreign government that were sold during the period.

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- Definition

Represents the aggregate amount of realized and unrealized gains and (losses) on capital support agreements, relating to certain proprietary liquidity funds. Amounts include the contractual obligations and benefits relating to the change in fair value of supported assets.

+ Details

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- Definition

Represents the aggregate amount of realized and unrealized gains and (losses) after tax, relating to certain proprietary liquidity funds. Amounts include the contractual obligations and benefits relating to the change in fair value of supported assets.

+ Details

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- Definition

The cash outflow for the reporting period associated with purchasing securities issued by Structured Investment Vehicles (SIV's), including letters of credit, total return swaps, and capital support agreements. Amounts are net of any distribution received from SIV's.

+ Details

Name:	lm_FundSupportPurchasesOfStructuredInvestmentVehicleSecuritiesNetOfDistributions
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Represents interest payments received on previously owned SIV securities.

+ Details

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Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Details

Name:	lm_LiquidityFundsSupportLineItems
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- Definition

Represents the amount of loss related to reimbursements to two funds for the portion of loss incurred in selling SIV securities.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Represents the accrued interest included in non-bank sponsored SIV securities purchased by the entity.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the principal amount of non-bank sponsored SIV securities purchased by the entity.

+ Details

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Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the number of capital support agreements the reporting entity entered into during the period.

+ Details

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- Definition

Represents the number of liquidity funds that were previously supported under CSAs and LOCs from which SIV securities were purchased.

+ Details

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- Definition

Number of liquidity funds reimbursed for losses.

+ Details

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- Definition

Proceeds from sale of previously supported securities and Canadian conduit securities to third parties, excluding transaction costs.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate amount of expense (excess cost over sale proceeds and related costs) associated with the realized gain (loss) on the sales of SIV securities during the period.

+ Details

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- Definition

Represents the amount of realized gains or losses, net of taxes and operating expense adjustments, associated with the sale of SIV securities.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Represents the amount of securities sold that were previously supported by a total return swap.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

The aggregate amount of unrealized gains and losses related to purchased SIV securities and supported non-asset backed securities.

+ Details

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Namespace Prefix:	lm
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Balance Type:	credit
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- Definition

The aggregate amount of unrealized gains or losses, net of taxes and operating expense adjustments, related to purchased SIV securities and supported non-asset backed securities.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Aggregate realized and unrealized gain on foreign currency transactions included in net income. Includes gain on a foreign currency forward exchange contract. Does not include foreign currency transactions designated as hedges of net investment in a foreign entity or intercompany foreign currency transactions that are of a long-term nature. Gross of tax effect.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 52
-Paragraph 30

+ Details

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