Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2013	Mar. 31, 2014
Accounting Policies [Abstract]
Investment in Proprietary Fund Products, Initial Investment		100.00%
Employee Owned Funds		16
Investment in Consolidated Sponsored Investment Funds	$ 39,056	$ 39,434
Cash and Cash Equivalents, Maturity Period	90
Threshold for Equity Method Accounting, Proprietary Fund Products, Low End		20.00%
Threshold for Equity Method Accounting, Proprietary Fund Products, High End		50.00%
Threshold for Equity Method Accounting, Partnership and Limited Liability Company Investments		3.00%
Long-term Debt, Fair Value	$ 1,206,166	$ 1,135,103

Represents the maximum original maturity of cash and cash equivalents, in days.

No definition available.

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

- Definition

Number of funds that are seeded by employee's in which Legg Mason is the primary beneficiary.

+ References

No definition available.

+ Details

Name:	lm_EmployeeOwnedFunds
Namespace Prefix:	lm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents the aggregate amount of the company's investment in the sponsored investment funds.

+ References

No definition available.

+ Details

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

- Definition

Typical initial investment percentage in proprietary fund product investments.

+ References

No definition available.

+ Details

Name:	lm_InvestmentinProprietaryFundProductsInitialInvestment
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage ownership used to determine if an investment in a partnership or limited liability company should be accounted for using the equity method of accounting

+ References

No definition available.

+ Details

Name:	lm_ThresholdforEquityMethodAccountingPartnershipandLimitedLiabilityCompanyInvestments
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage ownership used to determine if an investment in a proprietary fund product should be accounted for using the equity method of accounting - high end

+ References

No definition available.

+ Details

Name:	lm_ThresholdforEquityMethodAccountingProprietaryFundProductsHighEnd
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage ownership used to determine if an investment in a proprietary fund product should be accounted for using the equity method of accounting - low end

+ References

No definition available.

+ Details

Name:	lm_ThresholdforEquityMethodAccountingProprietaryFundProductsLowEnd
Namespace Prefix:	lm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Definition

The fair value amount of long-term debt 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.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant