Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified		3 Months Ended
	May 22, 2012	Mar. 31, 2012 Quarterly Financial Data [Member]	Dec. 31, 2011 Quarterly Financial Data [Member]	Sep. 30, 2011 Quarterly Financial Data [Member]	Jun. 30, 2011 Quarterly Financial Data [Member]	Mar. 31, 2011 Quarterly Financial Data [Member]	Dec. 31, 2010 Quarterly Financial Data [Member]	Sep. 30, 2010 Quarterly Financial Data [Member]	Jun. 30, 2010 Quarterly Financial Data [Member]
Operating Revenues		$ 648,591	$ 626,978	$ 669,897	$ 717,108	$ 713,430	$ 721,928	$ 674,794	$ 674,165
Operating Expenses		576,379	567,655	563,045	616,742	614,290	624,936	586,895	571,388
Operating Income		72,212	59,323	106,852	100,366	99,140	96,992	87,899	102,777
Other Non-operating Income (Expense)		37,781	(11,575)	(51,075)	(10,801)	3,486	(9,836)	15,409	(30,670)
Income before Income Tax Provision (Benefit)		109,993	47,748	55,777	89,565	102,626	87,156	103,308	72,107
Income tax provision (benefit)		33,184	12,607	(1,606)	27,867	31,858	33,792	26,720	27,064
Net Income		76,809	35,141	57,383	61,698	70,768	53,364	76,588	45,043
Less: Net income (loss) attributable to noncontrolling interests		740	7,009	719	1,746	1,731	(8,256)	1,253	(2,888)
Net income (loss) attributable to Legg Mason, Inc.		76,069	28,132	56,664	59,952	69,037	61,620	75,335	47,931
Net Income per Share attributable to Legg Mason, Inc. common shareholders:
Basic (in dollars per share)		$ 0.54	$ 0.20	$ 0.39	$ 0.40	$ 0.45	$ 0.41	$ 0.50	$ 0.30
Diluted (in dollars per share)		$ 0.54	$ 0.20	$ 0.39	$ 0.40	$ 0.45	$ 0.41	$ 0.50	$ 0.30
Cash dividend per share (in dollars per share)		$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.06	$ 0.06	$ 0.04	$ 0.04
Stock price range:
High (in dollars per share)		$ 29.49	$ 29.56	$ 34.32	$ 37.82	$ 37.29	$ 37.72	$ 31.04	$ 34.83
Low (in dollars per share)		$ 23.75	$ 22.61	$ 24.11	$ 30.86	$ 32.21	$ 29.68	$ 24.94	$ 27.36
Assets Under Management:
End of period		643,318	626,960	611,794	662,533	677,646	671,799	673,467	645,362
Average		$ 634,916	$ 622,004	$ 643,296	$ 670,761	$ 673,495	$ 672,399	$ 658,585	$ 668,268
Closing price of common stock (in dollars per share)	$ 24.39