EMPLOYEE BENEFITS (Details) (USD $)	12 Months Ended
Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
EMPLOYEE BENEFITS
Types of discretionary contributions	2
Percentage match of employee contribution under 401(k) plan (as a percent)	50.00%
Maximum contribution match by the employer as a percentage of employee compensation (as a percent)	6.00%
Maximum amount of contribution made by the employer per year	$ 5,000
Profit sharing and matching contributions	22,739,000	18,199,000
Matching contributions			$ 14,366,000

- Definition

This element represents the maximum limit of employer contribution to an employee's defined contribution account as a percentage of the employee's qualified compensation.

+ Details

Name:	lm_DefinedContributionPlanEmployerContributionMatchingPercentOfEligibleCompensation
Namespace Prefix:	lm
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the maximum percentage of employee contributions up to a specified percentage of compensation that will be matched by the employer.

+ Details

Name:	lm_DefinedContributionPlanEmployerContributionMatchingPercentOfEmployeeContribution
Namespace Prefix:	lm
Data Type:	us-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of matching contributions made under 401 (k) defined contribution plans.

+ Details

Name:	lm_DefinedContributionPlanMatchingContributions
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of maximum contribution made by employer under defined contribution plan per year.

+ Details

Name:	lm_DefinedContributionPlanMaximumEmployerContribution
Namespace Prefix:	lm
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Definition

Represents the number of types of discretionary contributions.

+ Details

Name:	lm_NumberOfTypesOfDiscretionaryContributions
Namespace Prefix:	lm
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 11

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration