Variable Interest Entities and Consolidation of Investment Vehicles (Details 2) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Before Consolidation [Member]
Total operating revenues	$ 670,522	$ 675,753	$ 1,388,682	$ 1,350,697
Total operating expenses	562,785	586,393	1,179,417	1,157,317
Operating Income	107,737	89,360	209,265	193,380
Total other non-operating income (expense)	(52,597)	12,746	(66,229)	(16,225)
Income Before Income Tax Provision	55,140	102,106	143,036	177,155
Income tax (benefit) provision	(1,606)	26,720	26,261	53,784
Net Income	56,746	75,386	116,775	123,371
Less: Net income (loss) attributable to noncontrolling interests	82	51	159	105
Net Income (Loss) Attributable to Parent	56,664	75,335	116,616	123,266
Consolidated Investment Vehicles
Total operating revenues	0	0	0	0
Total operating expenses	885	1,461	2,047	2,704
Operating Income	(885)	(1,461)	(2,047)	(2,704)
Total other non-operating income (expense)	3,081	4,499	8,183	2,106
Income Before Income Tax Provision	2,196	3,038	6,136	(598)
Income tax (benefit) provision	0	0	0	0
Net Income	2,196	3,038	6,136	(598)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
Net Income (Loss) Attributable to Parent	2,196	3,038	6,136	(598)
Eliminations
Total operating revenues	(625)	(959)	(1,677)	(1,738)
Total operating expenses	(625)	(959)	(1,677)	(1,738)
Operating Income	0	0	0	0
Total other non-operating income (expense)	(1,559)	(1,836)	(3,830)	(1,142)
Income Before Income Tax Provision	(1,559)	(1,836)	(3,830)	(1,142)
Income tax (benefit) provision	0	0	0	0
Net Income	(1,559)	(1,836)	(3,830)	(1,142)
Less: Net income (loss) attributable to noncontrolling interests	637	1,202	2,306	(1,740)
Net Income (Loss) Attributable to Parent	(2,196)	(3,038)	(6,136)	598
Consolidated Legg Mason Group [Member]
Total operating revenues	669,897	674,794	1,387,005	1,348,959
Total operating expenses	563,045	586,895	1,179,787	1,158,283
Operating Income	106,852	87,899	207,218	190,676
Total other non-operating income (expense)	(51,075)	15,409	(61,876)	(15,261)
Income Before Income Tax Provision	55,777	103,308	145,342	175,415
Income tax (benefit) provision	(1,606)	26,720	26,261	53,784
Net Income	57,383	76,588	119,081	121,631
Less: Net income (loss) attributable to noncontrolling interests	719	1,253	2,465	(1,635)
Net Income (Loss) Attributable to Parent	$ 56,664	$ 75,335	$ 116,616	$ 123,266