The number of market-based performance shares outstanding during the period.

No definition available.

Name:	lm_RemainingPerformanceSharesOutstanding
Namespace Prefix:	lm_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the beginning stock price hurdle in year one.

+ References

No definition available.

+ Details

Name:	lm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsStockPriceHurdlesBeginningInYearOne
Namespace Prefix:	lm_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the ending stock price hurdle in year five.

+ References

No definition available.

+ Details

Name:	lm_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsStockPriceHurdlesEndingInYearFive
Namespace Prefix:	lm_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the grant price per share of awards based on which the weighted-average fair value per share of awards is determined.

+ References

No definition available.

+ Details

Name:	lm_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsGrantPrice
Namespace Prefix:	lm_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	lm_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsLineItems
Namespace Prefix:	lm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

-URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)(a)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Stock-Based Compensation (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	1 Months Ended	12 Months Ended		12 Months Ended
	Jan. 31, 2008 Market-based performance shares	Mar. 31, 2012 Market-based performance shares	Jan. 28, 2008 Market-based performance shares	Mar. 31, 2012 Black-Scholes option pricing model	Mar. 31, 2011 Black-Scholes option pricing model	Mar. 31, 2010 Black-Scholes option pricing model
Share-based Compensation Arrangement by Share Based Payment Award Fair Value Assumptions
Expected dividend yield (as a percent)		1.33%		1.39%	1.39%	1.45%
Risk-free interest rate (as a percent)		3.30%		1.95%	2.37%	2.86%
Expected volatility (as a percent)		36.02%		47.16%	52.64%	55.26%
Expected lives (in years)				5.12	5.18	5.17
Granted (in shares)	120
Remaining Performance Shares Outstanding		100
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments, Other than Options, Stock Price Hurdles Beginning in Year One			$ 77.97
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments, Other than Options, Stock Price Hurdles Ending in Year Five			$ 114.15
Granted (in dollars per share)		$ 11.81
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Grant Price		$ 70.88