VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES (Details 2) (USD $) In Thousands	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
Total 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
Total 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 (Loss)	99,140	96,992	87,899	102,777	105,836	79,148	77,884	58,315	386,808	321,183	(669,180)
Total 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 (loss) 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 benefit (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 (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)
Balance Before Consolidation of CIVs
Total operating revenues									2,788,450	2,637,658	3,358,599
Total operating expenses									2,396,938	2,314,376	4,025,842
Operating Income (Loss)									391,512	323,282	(667,243)
Total other non-operating income (expense)									(17,931)	(47)	(2,523,722)
Income (loss) before income tax provision									373,581	323,235	(3,190,965)
Income tax benefit (provision)									119,434	118,676	(1,223,203)
Net income (loss)									254,147	204,559	(1,967,762)
Less: Net income (loss) attributable to noncontrolling interests									224	202	156
Net income (loss) attributable to Legg Mason, Inc.									253,923	204,357	(1,967,918)
CIVs
Total operating expenses									4,704	2,263	1,938
Operating Income (Loss)									(4,704)	(2,263)	(1,938)
Total other non-operating income (expense)									1,704	17,329	7,796
Income (loss) before income tax provision									(3,000)	15,066	5,858
Net income (loss)									(3,000)	15,066	5,858
Net income (loss) attributable to Legg Mason, Inc.									(3,000)	15,066	5,858
Eliminations
Total operating revenues									(4,133)	(2,779)	(1,232)
Total operating expenses									(4,133)	(2,943)	(1,233)
Operating Income (Loss)										164	1
Total other non-operating income (expense)									(5,384)	(8,809)	(3,091)
Income (loss) before income tax provision									(5,384)	(8,645)	(3,090)
Net income (loss)									(5,384)	(8,645)	(3,090)
Less: Net income (loss) attributable to noncontrolling interests									(8,384)	6,421	2,768
Net income (loss) attributable to Legg Mason, Inc.									$ 3,000	$ (15,066)	$ (5,858)