Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended					3 Months Ended
	Sep. 30, 2009	Mar. 31, 2012	Mar. 31, 2010	Mar. 31, 2012 Consolidated Legg Mason, Inc.	Mar. 31, 2011 Consolidated Legg Mason, Inc.	Mar. 31, 2010 Consolidated Legg Mason, Inc.	Jun. 30, 2011 5.6% senior notes from Equity Units Consolidated Legg Mason, Inc.
Earnings per share
Stock Repurchased and Retired During Period, Shares				13,597	14,552	0
Shares repurchased and retired				$ 400,266	$ 445,465
Repurchased and Retired Shares, Excluded from Weighted Average Shares Outstanding Number				9,716	9,088
Common stock issued through Equity Units transactions (in shares)	18,596						1,830
Common stock issued through Equity Units included in weighted-average shares outstanding (in shares)		1,380	11,565
Weighted-average basic shares outstanding (in shares)				143,292	155,321	153,715
Potential common shares:
Employee stock options (in shares)				57	163	56
Shares related to deferred compensation (in shares)				0	0	455
Shares issuable upon payment of contingent consideration (in shares)				0	0	1,136
Weighted-average diluted shares (in shares)				143,349	155,484	155,362
Net Income				231,031	245,763	210,980
Less: Net income (loss) attributable to noncontrolling interests				10,214	(8,160)	6,623
Net income (loss) attributable to Legg Mason, Inc.				$ 220,817	$ 253,923	$ 204,357
Basic (in dollars per share)				$ 1.54	$ 1.63	$ 1.33
Diluted (in dollars per share)				$ 1.54	$ 1.63	$ 1.32