Variable Interest Entities and Consolidation of Investment Vehicles (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2010
Before Consolidation [Member]
Total operating revenues	$ 627,731	$ 723,087	$ 2,016,413	$ 2,073,784
Total operating expenses	567,550	625,452	1,746,967	1,782,769
Operating Income	60,181	97,635	269,446	291,015
Total other non-operating income (expense)	(19,357)	(2,177)	(85,586)	(18,402)
Income Before Income Tax Provision	40,824	95,458	183,860	272,613
Income tax (benefit) provision	12,607	33,792	38,868	87,576
Net Income	28,217	61,666	144,992	185,037
Less: Net income (loss) attributable to noncontrolling interests	85	46	244	151
Net Income (Loss) Attributable to Parent	28,132	61,620	144,748	184,886
Consolidated Investment Vehicles
Total operating revenues	0	0	0	0
Total operating expenses	859	643	2,906	3,347
Operating Income	(859)	(643)	(2,906)	(3,347)
Total other non-operating income (expense)	7,424	(8,462)	15,607	(6,356)
Income Before Income Tax Provision	6,565	(9,105)	12,701	(9,703)
Income tax (benefit) provision	0	0	0	0
Net Income	6,565	(9,105)	12,701	(9,703)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
Net Income (Loss) Attributable to Parent	6,565	(9,105)	12,701	(9,703)
Eliminations
Total operating revenues	(753)	(1,159)	(2,430)	(2,897)
Total operating expenses	(754)	(1,159)	(2,431)	(2,897)
Operating Income	1	0	1	0
Total other non-operating income (expense)	358	803	(3,472)	(339)
Income Before Income Tax Provision	359	803	(3,471)	(339)
Income tax (benefit) provision	0	0	0	0
Net Income	359	803	(3,471)	(339)
Less: Net income (loss) attributable to noncontrolling interests	6,924	(8,302)	9,230	(10,042)
Net Income (Loss) Attributable to Parent	(6,565)	9,105	(12,701)	9,703
Consolidated Legg Mason Group [Member]
Total operating revenues	626,978	721,928	2,013,983	2,070,887
Total operating expenses	567,655	624,936	1,747,442	1,783,219
Operating Income	59,323	96,992	266,541	287,668
Total other non-operating income (expense)	(11,575)	(9,836)	(73,451)	(25,097)
Income Before Income Tax Provision	47,748	87,156	193,090	262,571
Income tax (benefit) provision	12,607	33,792	38,868	87,576
Net Income	35,141	53,364	154,222	174,995
Less: Net income (loss) attributable to noncontrolling interests	7,009	(8,256)	9,474	(9,891)
Net Income (Loss) Attributable to Parent	$ 28,132	$ 61,620	$ 144,748	$ 184,886