Fair Values of Assets and Liabilities (Details) (Legg Mason, Inc., USD $) In Thousands	Jun. 30, 2011	Mar. 31, 2011
Legg Mason, Inc.
Equity method investments of deferred compensation plans	$ 39,613	$ 48,528

Name:

lm_EquityMethodInvestmentsDeferredCompensationPlanAssets

Namespace Prefix:

lm

Data Type:

xbrli:monetaryItemType

Balance Type:

debit

Period Type:

instant