Long-Term Debt and Equity Units (Details 3) (USD $) In Thousands, unless otherwise specified		3 Months Ended		6 Months Ended			3 Months Ended			6 Months Ended
	Sep. 30, 2011	Sep. 30, 2011 Legg Mason, Inc. 2.5% convertible senior notes	Sep. 30, 2010 Legg Mason, Inc. 2.5% convertible senior notes	Sep. 30, 2011 Legg Mason, Inc. 2.5% convertible senior notes	Sep. 30, 2010 Legg Mason, Inc. 2.5% convertible senior notes	Mar. 31, 2011 Legg Mason, Inc. 2.5% convertible senior notes	Jun. 30, 2011 Legg Mason, Inc. 5.6% senior notes from Equity Units	Sep. 30, 2011 Legg Mason, Inc. 5.6% senior notes from Equity Units	Mar. 31, 2011 Legg Mason, Inc. 5.6% senior notes from Equity Units	Sep. 30, 2011 Legg Mason, Inc.	Sep. 30, 2010 Legg Mason, Inc.	Mar. 31, 2011 Legg Mason, Inc.
Debt Instrument [Line Items]
Long-term Debt, Fair Value										$ 1,259,200
Amount by which the accreted value of the Notes exceeds their if-converted value				141,532
Long-term debt		1,107,162		1,107,162		1,087,932		0	103,039	1,117,668		1,201,868
Debt instrument interest rate stated percentage (as a percent)	2.50%	2.50%		2.50%			5.60%
Debt instrument, effective borrowing rate used to accrete carrying value (as a percent)				6.50%
Imputed Interest for 2.5% Convertible Senior Notes		$ 9,741	$ 9,146	$ 19,230	$ 18,055					$ 19,230	$ 18,055
Debt instrument term for computation of effective rate (in years)				7
Percentage of interest rate used in the calculation of the if-converted value of convertible debt (as a percent)				2.70%
Equity Units exchange							1,830