EARNINGS PER SHARE (Details) (USD $) In Thousands, except Per Share data	1 Months Ended	3 Months Ended								12 Months Ended
	Aug. 31, 2009	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2009	Jun. 30, 2009	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
EARNINGS PER SHARE
Common stock purchased and retired (in shares)										14,552
Common stock purchased and retired excluded from weighted-average shares outstanding (in shares)										9,088
Common stock issued through Equity Units tender offer (in shares)	18,596
Common stock issued through Equity Units included in weighted-average shares outstanding (in shares)	11,565
Weighted-average basic shares outstanding (in shares)										155,321	153,715	140,669
Potential common shares:
Employee stock options (in shares)										163	56
Shares related to deferred compensation (in shares)											455
Shares issuable upon payment of contingent consideration (in shares)											1,136
Weighted-average diluted shares (in shares)										155,484	155,362	140,669
Net income (loss)		$ 70,768	$ 53,364	$ 76,588	$ 45,043	$ 65,101	$ 46,233	$ 47,322	$ 52,324	$ 245,763	$ 210,980	$ (1,964,994)
Less: Net income (loss) attributable to noncontrolling interests		1,731	(8,256)	1,253	(2,888)	1,494	1,311	1,548	2,270	(8,160)	6,623	2,924
Net income (loss) attributable to Legg Mason, Inc.		$ 69,037	$ 61,620	$ 75,335	$ 47,931	$ 63,607	$ 44,922	$ 45,774	$ 50,054	$ 253,923	$ 204,357	$ (1,967,918)
Net Income per Share Attributable to Legg Mason, Inc. common shareholders, Basic (in dollars per share)		$ 0.45	$ 0.41	$ 0.50	$ 0.30	$ 0.40	$ 0.28	$ 0.30	$ 0.35	$ 1.63	$ 1.33	$ (13.99)
Net Income per Share Attributable to Legg Mason, Inc. common shareholders, Diluted (in dollars per share)		$ 0.45	$ 0.41	$ 0.50	$ 0.30	$ 0.39	$ 0.28	$ 0.30	$ 0.35	$ 1.63	$ 1.32	$ (13.99)