Consolidated Statements of Changes in Stockholders' Equity (Consolidated Legg Mason [Domain], USD $) In Thousands, unless otherwise specified	Consolidated Legg Mason [Domain] USD ($)	Consolidated Legg Mason [Domain] Common Stock USD ($)	Consolidated Legg Mason [Domain] ADDITIONAL PAID-IN CAPITAL USD ($)	Consolidated Legg Mason [Domain] EMPLOYEE STOCK TRUST USD ($)	Consolidated Legg Mason [Domain] DEFERRED COMPENSATION EMPLOYEE STOCK TRUST USD ($)	Consolidated Legg Mason [Domain] RETAINED EARNINGS USD ($)	Consolidated Legg Mason [Domain] APPROPRIATED RETAINED EARNINGS FOR CONSOLIDATED INVESTMENT VEHICLE USD ($)	Consolidated Legg Mason [Domain] ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET USD ($)
Balance at Mar. 31, 2011		$ 15,022	$ 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
Equity Units exchanged		183	102,831
Employee tax withholdings by net share transactions		0	0
Shares repurchased and retired		(1,360)	(398,906)
Allocation from 2.5% Convertible Senior Notes repurchase, net of tax			0
Shares issued to plans				(1,481)	1,481
Distributions and forfeitures				914	(914)
Net income (loss) attributable to Legg Mason, Inc.	116,616					116,616
Dividends declared						(23,695)
Net income (loss) reclassified to appropriated retained earnings							(5,802)
Net unrealized holding gains (losses) on investment securities	257							257
Foreign currency translation adjustment	(32,666)							(32,666)
Balance at Sep. 30, 2011	5,555,737	13,976	3,842,784	(35,033)	35,033	1,632,905	5,120	60,952
Balance at Jun. 30, 2011
Increase (Decrease) in Stockholders' Equity
Net income (loss) attributable to Legg Mason, Inc.	56,664
Net unrealized holding gains (losses) on investment securities	186
Foreign currency translation adjustment	(46,026)
Balance at Sep. 30, 2011	5,555,737
Balance at Mar. 31, 2012	5,677,291	13,987	3,864,216	(32,419)	32,419	1,715,395	12,221	71,472
Increase (Decrease) in Stockholders' Equity
Stock options and other stock-based compensation		6	7,173
Deferred compensation employee stock trust		5	1,232
Deferred compensation, net		175	20,846
Equity Units exchanged		0	0
Employee tax withholdings by net share transactions		(29)	(7,632)
Shares repurchased and retired		(973)	(244,098)
Allocation from 2.5% Convertible Senior Notes repurchase, net of tax			(30,833)
Shares issued to plans				(1,237)	1,237
Distributions and forfeitures				411	(411)
Net income (loss) attributable to Legg Mason, Inc.	71,339					71,339
Dividends declared						(29,550)
Net income (loss) reclassified to appropriated retained earnings							(3,046)
Net unrealized holding gains (losses) on investment securities	63							63
Foreign currency translation adjustment	(5,469)							(5,469)
Balance at Sep. 30, 2012	5,456,500	13,171	3,610,904	(33,245)	33,245	1,757,184	9,175	66,066
Balance at Jun. 30, 2012
Increase (Decrease) in Stockholders' Equity
Net income (loss) attributable to Legg Mason, Inc.	80,797
Net unrealized holding gains (losses) on investment securities	10
Foreign currency translation adjustment	7,666
Balance at Sep. 30, 2012	$ 5,456,500