CAPITAL STOCK (Details) (USD $)	1 Months Ended	12 Months Ended					1 Months Ended	12 Months Ended			12 Months Ended
	May 31, 2010	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009	Mar. 31, 2011 Legg Mason, Inc. 2.5% convertible senior notes	Jan. 31, 2008 Legg Mason, Inc. 2.5% convertible senior notes	May 31, 2008 Legg Mason, Inc. 5.6% senior notes from Equity Units	Mar. 31, 2011 Legg Mason, Inc. 5.6% senior notes from Equity Units	Mar. 31, 2010 Legg Mason, Inc. 5.6% senior notes from Equity Units	Sep. 30, 2009 Legg Mason, Inc. 5.6% senior notes from Equity Units	Mar. 31, 2011 COMMON STOCK	Mar. 31, 2010 COMMON STOCK	Mar. 31, 2009 COMMON STOCK	Mar. 31, 2011 SHARES EXCHANGEABLE INTO COMMON STOCK	Mar. 31, 2010 SHARES EXCHANGEABLE INTO COMMON STOCK	Mar. 31, 2009 SHARES EXCHANGEABLE INTO COMMON STOCK
CAPITAL STOCK
Common stock, authorized shares		500,000,000	500,000,000
Preferred stock, authorized shares		4,000,000
Shares of common stock reserved for issuance under equity plans		14,557,000	16,377,000
Common shares reserved for exchangeable shares issued in connection with acquisition			1,099,000
Stock buyback authority, maximum amount (in dollars)	$ 1,000,000,000
Number of counterparties in accelerated share repurchase agreements		2
Common stock to be repurchased under accelerated share repurchase agreements (in dollars)		300,000,000
Shares of common stock received under the ASR Agreements		10,147,000
Shares repurchased on the open market and retired		4,405,000
Shares repurchased on the open market and retired (in dollars)		145,067,000
Stock buyback authority, remaining amount (in dollars)		554,933,000
Proceeds from issuance of equity units (in dollars)							1,150,000,000
Percent of interest in $1,000 principal amount of senior notes per equity unit issued (as a percent)								5.00%
Debt component of equity unit, principal amount of Note per unit								1,000
Percentage of equity units exchanged (as a percent)									91.00%
Maximum number of shares of common stock that could be issued, and are reserved for issuance						14,205,000		1,830,000
Principal amount of debt issued (in dollars)						1,250,000,000	1,000
Debt instrument interest rate stated percentage (as a percent)					2.50%	2.50%		5.60%
SHARES EXCHANGEABLE INTO COMMON STOCK
Beginning balance														1,099,000	1,222,000	1,983,000
Exchanges														(1,099,000)	(123,000)	(761,000)
Ending balance															1,099,000	1,222,000
COMMON STOCK
Beginning balance		161,438,993									161,439,000	141,853,000	138,556,000
Shares issued for:
Stock option exercises and other stock-based compensation											638,000	72,000	1,094,000
Deferred compensation trust											75,000	133,000	155,000
Deferred compensation											1,520,000	662,000	922,000
Exchangeable shares											1,099,000	123,000	761,000
Shares repurchased and retired		(14,552,000)									(14,552,000)
Conversion of non-voting preferred stock													365,000
Equity Units exchange										18,596,000		18,596,000
Ending balance		150,218,810	161,438,993								150,219,000	161,439,000	141,853,000
Dividends Declared per Share (in dollars per share)		$ 0.20	$ 0.12	$ 0.96
Dividends declared but not paid (in dollars)		$ 8,990,000	$ 4,844,000	$ 34,043,000