CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Consolidated Legg Mason Group [Member], USD $) In Thousands	Consolidated Legg Mason Group [Member] USD ($)	Consolidated Legg Mason Group [Member] COMMON STOCK USD ($)	Consolidated Legg Mason Group [Member] SHARES EXCHANGEABLE INTO COMMON STOCK USD ($)	Consolidated Legg Mason Group [Member] ADDITIONAL PAID-IN CAPITAL USD ($)	Consolidated Legg Mason Group [Member] EMPLOYEE STOCK TRUST USD ($)	Consolidated Legg Mason Group [Member] DEFERRED COMPENSATION EMPLOYEE STOCK TRUST USD ($)	Consolidated Legg Mason Group [Member] RETAINED EARNINGS USD ($)	Consolidated Legg Mason Group [Member] APPROPRIATED RETAINED EARNINGS OF CONSOLIDATED INVESTMENT VEHICLES USD ($)	Consolidated Legg Mason Group [Member] ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET USD ($)
Balance at Mar. 31, 2010		$ 16,144	$ 2,760	$ 4,447,612	$ (33,095)	$ 33,095	$ 1,316,981	$ 0	$ 58,227
Increase (Decrease) in Stockholders' Equity
Stock options and other stock-based compensation		35		18,438
Deferred compensation employee stock trust		5		1,448
Deferred compensation, net		141		16,138
Exchangeable shares		110	(2,760)	2,650
Equity Units exchanged		0		0
Shares repurchased and retired		(1,139)		(335,262)
Shares issued to plans					(1,301)	1,301
Distributions and forfeitures					249	(249)
Net Income (Loss) Attributable to Parent	123,266						123,266
Dividends declared							(12,737)
Cumulative effect of change in accounting principle								24,666
Net loss reclassified to appropriated retained earnings								(2,650)
Unrealized holding gains on investment securities, net of tax	82								82
Foreign currency translation adjustment	19,972								19,972
Balance at Sep. 30, 2010	5,694,127	15,296	0	4,151,024	(34,147)	34,147	1,427,510	22,016	78,281
Balance at Jun. 30, 2010
Increase (Decrease) in Stockholders' Equity
Net Income (Loss) Attributable to Parent	75,335
Unrealized holding gains on investment securities, net of tax	20
Foreign currency translation adjustment	29,588
Balance at Sep. 30, 2010	5,694,127
Balance at Mar. 31, 2011	5,770,384	15,022	0	4,111,095	(34,466)	34,466	1,539,984	10,922	93,361
Increase (Decrease) in Stockholders' Equity
Stock options and other stock-based compensation		7		9,800
Deferred compensation employee stock trust		4		1,477
Deferred compensation, net		120		16,487
Exchangeable shares		0	0	0
Equity Units exchanged		183		102,831
Shares repurchased and retired	(400,266)	(1,360)		(398,906)
Shares issued to plans					(1,481)	1,481
Distributions and forfeitures					914	(914)
Net Income (Loss) Attributable to Parent	116,616						116,616
Dividends declared							(23,695)
Cumulative effect of change in accounting principle								0
Net loss reclassified to appropriated retained earnings								(5,802)
Unrealized holding gains on investment securities, net of tax	257								257
Foreign currency translation adjustment	(32,666)								(32,666)
Balance at Sep. 30, 2011	5,555,737	13,976	0	3,842,784	(35,033)	35,033	1,632,905	5,120	60,952
Balance at Jun. 30, 2011
Increase (Decrease) in Stockholders' Equity
Shares repurchased and retired	(200,147)
Net Income (Loss) Attributable to Parent	56,664
Unrealized holding gains on investment securities, net of tax	186
Foreign currency translation adjustment	(46,026)
Balance at Sep. 30, 2011	$ 5,555,737