Significant Accounting Policies (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Variable Interest Entity [Line Items]
Number of reportable segments	1	2
Number of operating segments	1
Legg Mason, Inc.
Variable Interest Entity [Line Items]
Assets Issued by Collateralized Loan Obligations	291,853	314,617
Long-term debt of consolidated investment vehicles	271,707	278,320
Equipment
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, Useful Life, Maximum	8
Property, Plant and Equipment, Useful Life, Minimum	3
Software
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment, Useful Life, Average	3

Sum of the carrying values as of the balance sheet date of all assets issued by collateralized loan obligations.

No definition available.

Name:	lm_AssetsIssuedByCollateralizedLoanObligations
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sum of the carrying values as of the balance sheet date of all long-term debt of the consolidated investment vehicles, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year (current maturities) or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

+ References

No definition available.

+ Details

Name:	lm_NoncurrentLongTermDebtOfConsolidatedInvestmentVehicles
Namespace Prefix:	lm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The reporting entity's number of operating segments.

+ References

No definition available.

+ Details

Name:	lm_NumberOfOperatingSegments
Namespace Prefix:	lm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The reporting entity's number of reportable segments, which are operating segments that meet the segment reporting criteria set forth in the related guidance.

+ References

No definition available.

+ Details

Name:	lm_NumberOfReportableSegments
Namespace Prefix:	lm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The average useful life of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLifeAverage
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum useful life of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLifeMaximum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The minimum useful life of long lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLifeMinimum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration