Income Taxes Income Taxes (Details 5) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Mar. 31, 2012
8. Income Taxes [Abstract]
Accumulated Earnings Planned to be Repatriated, Total	$ 225,000
Earnings from foreign subsidiaries that have been repatriated	100,000
Intend to Repatriate, Minimum	100,000
Intend to Repatriate, Maximum	$ 150,000

Represents the amount of earnings of foreign subsidiaries repatriated.

No definition available.

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