Consolidated Statements of Changes in Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Consolidated Legg Mason [Domain]	Consolidated Legg Mason [Domain] Common Stock	Consolidated Legg Mason [Domain] ADDITIONAL PAID-IN CAPITAL	Consolidated Legg Mason [Domain] EMPLOYEE STOCK TRUST	Consolidated Legg Mason [Domain] DEFERRED COMPENSATION EMPLOYEE STOCK TRUST	Consolidated Legg Mason [Domain] RETAINED EARNINGS	Consolidated Legg Mason [Domain] APPROPRIATED RETAINED EARNINGS FOR CONSOLIDATED INVESTMENT VEHICLE	Consolidated Legg Mason [Domain] ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET
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	13,465
Deferred compensation employee stock trust			6	1,901
Deferred compensation, net			119	25,357
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					(1,907)	1,907
Distributions and forfeitures					1,671	(1,671)
Net income (loss) attributable to Legg Mason, Inc.		144,748					144,748
Dividends declared							(34,019)
Net income (loss) reclassified to appropriated retained earnings								1,355
Net unrealized holding gains (losses) on investment securities		258							258
Foreign currency translation adjustment		(29,357)							(29,357)
Balance at Dec. 31, 2011		5,596,972	13,977	3,855,743	(34,702)	34,702	1,650,713	12,277	64,262
Balance at Sep. 30, 2011
Increase (Decrease) in Stockholders' Equity
Net income (loss) attributable to Legg Mason, Inc.		28,132
Net unrealized holding gains (losses) on investment securities		1
Foreign currency translation adjustment		3,309
Balance at Dec. 31, 2011		5,596,972
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			7	9,890
Deferred compensation employee stock trust			6	1,592
Deferred compensation, net			175	31,554
Equity Units exchanged			0	0
Employee tax withholdings by net share transactions			(30)	(7,871)
Shares repurchased and retired	(316,624)		(1,251)	(315,372)
Allocation from 2.5% Convertible Senior Notes repurchase, net of tax				(30,833)
Shares issued to plans					(1,598)	1,598
Distributions and forfeitures					696	(696)
Net income (loss) attributable to Legg Mason, Inc.		(382,531)					(382,531)
Dividends declared							(43,693)
Net income (loss) reclassified to appropriated retained earnings								(8,076)
Net unrealized holding gains (losses) on investment securities		27							27
Foreign currency translation adjustment		(12,576)							(12,576)
Balance at Dec. 31, 2012		4,918,309	12,894	3,553,176	(33,321)	33,321	1,289,171	4,145	58,923
Balance at Sep. 30, 2012
Increase (Decrease) in Stockholders' Equity
Shares repurchased and retired	(71,553)
Net income (loss) attributable to Legg Mason, Inc.		(453,870)
Net unrealized holding gains (losses) on investment securities		(37)
Foreign currency translation adjustment		(7,107)
Balance at Dec. 31, 2012		$ 4,918,309