Earnings Per Share (Details) (Consolidated Legg Mason Group [Member], USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Consolidated Legg Mason Group [Member]
Earnings per share
Weighted-average basic shares outstanding (in shares)	143,877	151,416	146,529	155,746
Potential common shares:
Employee stock options (in shares)	54	111	96	143
Shares related to deferred compensation (in shares)	0	413	0	438
Weighted-average diluted shares (in shares)	143,931	151,940	146,625	156,327
Net Income	$ 57,383	$ 76,588	$ 119,081	$ 121,631
Less: Net income (loss) attributable to noncontrolling interests	719	1,253	2,465	(1,635)
Net Income (Loss) Attributable to Parent	$ 56,664	$ 75,335	$ 116,616	$ 123,266
Net Income per Share Attributable to Legg Mason, Inc. common shareholders, Basic (in dollars per share)	$ 0.39	$ 0.50	$ 0.80	$ 0.79
Net Income per Share Attributable to Legg Mason, Inc. common shareholders, Diluted (in dollars per share)	$ 0.39	$ 0.50	$ 0.80	$ 0.79