VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES (Details 6) (USD $) In Thousands	12 Months Ended
Mar. 31, 2011
CLO loans and bonds
Unpaid principal balance	$ 299,044
Unpaid principal balance in excess of fair value	(4,283)
Fair value	294,761
Unpaid principal balance of loans that are more than 90 days past due and also in nonaccrual status	4,963
Unpaid principal balance in excess of fair value for loans that are more than 90 days past due and also in nonaccrual status	(2,837)
Fair value of loans more than 90 days past due and in nonaccrual status	2,126
Debt.
Principal amounts outstanding	300,959
Excess unpaid principal over fair value	(22,639)
Fair value	278,320
Fair Value, Option, Changes in Fair Value, Gain (Loss) recognized in Other non-operting income (expense) of CIVs	$ (14,686)

Name:	lm_CLOLoansAbstract
Namespace Prefix:	lm
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized that are 90 days or more past due.

+ Details

Name:	lm_PrincipalAmountOutstandingOfLoansHeldInPortfolio90DaysOrMorePastDue
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Estimated fair value of the debt instrument at the balance-sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 107
-Paragraph 10

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of outstanding principal due under the debt instrument at the end of the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number APB14-1
-Paragraph 31
-Subparagraph b

+ Details

Name:	us-gaap_DebtInstrumentPrincipalOutstanding
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DebtInstrumentsAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of loans and long-term receivables (other than securities categorized as trading, available-for-sale or held-to-maturity) that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 18
-Subparagraph d(1)

+ Details

Name:	us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of long-term debt instruments that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 18
-Subparagraph d(2)

+ Details

Name:	us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents, for each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings during the period and in which line in the income statement those gains and losses are reported. This item may also include amounts of gains and losses for other items measured at fair value, but for which the fair value option has not been elected (for instance, items required to be measured at fair value).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 19
-Subparagraph a

+ Details

Name:	us-gaap_FairValueOptionChangesInFairValueGainLoss
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 18
-Subparagraph e(1)

+ Details

Name:	us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due, in nonaccrual status, or both.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 159
-Paragraph 18
-Subparagraph e(3)

+ Details

Name:	us-gaap_FairValueOptionLoansHeldAsAssetsAggregateDifference
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents loan receivable as presented on the statement of financial position which may include mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts, or any other loans which are due the Company as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 114
-Paragraph 4

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This is the result of the principal amount outstanding of total loans managed or securitized less securitized loans and loans that are in the process of being securitized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number FAS140-4 and FIN46(R)-8
-Paragraph B11
-Subparagraph b(6)

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 140
-Paragraph 17
-Subparagraph i(4)

+ Details

Name:	us-gaap_PrincipalAmountOutstandingOfLoansHeldInPortfolio
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant