QUARTERLY FINANCIAL DATA (Details) (USD $) In Thousands, except Per Share data	3 Months Ended	12 Months Ended
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	May 20, 2011
QUARTERLY FINANCIAL DATA
Operating Revenues	$ 713,430	$ 721,928	$ 674,794	$ 674,165	$ 671,420	$ 690,479	$ 659,896	$ 613,084	$ 2,784,317	$ 2,634,879	$ 3,357,367
Operating Expenses	614,290	624,936	586,895	571,388	565,584	611,331	582,012	554,769	2,397,509	2,313,696	4,026,547
Operating Income	99,140	96,992	87,899	102,777	105,836	79,148	77,884	58,315	386,808	321,183	(669,180)
Other Non-Operating Income (Expense)	3,486	(9,836)	15,409	(30,670)	(4,116)	(6,909)	(2,891)	22,389	(21,611)	8,473	(2,519,017)
Income before Income Tax Provision	102,626	87,156	103,308	72,107	101,720	72,239	74,993	80,704	365,197	329,656	(3,188,197)
Income tax provision	31,858	33,792	26,720	27,064	36,619	26,006	27,671	28,380	119,434	118,676	(1,223,203)
Net Income	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 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)
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)
Cash dividend per share (in dollars per share)	$ 0.06	$ 0.06	$ 0.04	$ 0.04	$ 0.03	$ 0.03	$ 0.03	$ 0.03
Stock price range:
High (in dollars per share)	$ 37.29	$ 37.72	$ 31.04	$ 34.83	$ 31.95	$ 33.70	$ 33.08	$ 26.74	$ 37.29	$ 31.95
Low (in dollars per share)	$ 32.21	$ 29.68	$ 24.94	$ 27.36	$ 24.00	$ 26.99	$ 22.06	$ 15.53	$ 32.21	$ 24.00
Assets Under Management:
End of period	677,646	671,799	673,467	645,362	684,549	681,614	702,700	656,857	677,646	684,549
Average	$ 673,495	$ 672,399	$ 658,585	$ 668,268	$ 681,227	$ 693,254	$ 684,034	$ 647,218
Closing price of common stock (in dollars per share)												$ 33.55

The market value of assets an investment adviser manages on behalf of its clients.

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Aggregate dividends paid during the period for each share of common stock outstanding.

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