Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands		Total	Consolidated Legg Mason, Inc.	Consolidated Legg Mason, Inc. COMMON STOCK	Consolidated Legg Mason, Inc. ADDITIONAL PAID-IN CAPITAL	Consolidated Legg Mason, Inc. EMPLOYEE STOCK TRUST	Consolidated Legg Mason, Inc. DEFERRED COMPENSATION EMPLOYEE STOCK TRUST	Consolidated Legg Mason, Inc. RETAINED EARNINGS	Consolidated Legg Mason, Inc. ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET	Royce [Member]	Clarion Partners [Member] Consolidated Legg Mason, Inc. ADDITIONAL PAID-IN CAPITAL	EnTrust Capital [Member] Consolidated Legg Mason, Inc. RETAINED EARNINGS	Management Equity Plan [Member] Consolidated Legg Mason, Inc. RETAINED EARNINGS	Management Equity Plan [Member] Royce [Member]
Stockholders' Equity Attributable to Parent				$ 10,701	$ 2,693,113	$ (26,263)	$ 26,263	$ 1,576,242	$ (66,493)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													$ 22,202
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest										$ 3,707				3,707
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		$ (10,338)	$ 13,061											(2,723)	[1]
Stock options exercised				12	3,442
Deferred compensation employee stock trust				1	254
Stock-based compensation				37	43,773						$ 11,121
Employee tax withholdings by settlement of net share transactions		(11,802)		(36)	(11,766)
Shares repurchased and retired		(201,673)		(617)	(201,056)
APIC reclassified for MEP vesting					(3,632)
Shares Issued to plans						(255)	255
Distributions and forfeitures						1,067	(1,067)
Net Income Attributable to Legg Mason, Inc.			99,893					99,893
Dividends declared								(46,039)
RE Reclassified for MEP Amortization		5,353						(1,721)
Excess Tax Benefit from Share-based Compensation, Financing Activities								0
Reclassification from Retained Earnings to Noncontrolling Interests												$ (15,500)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax									(19,159)
Foreign currency translation adjustment			(13,023)
Ending Balance at Sep. 30, 2016			4,082,733
Stockholders' Equity Attributable to Parent			4,059,547	10,098	2,535,249	(25,451)	25,451	1,612,875	(98,675)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													23,186
Stockholders' Equity Attributable to Parent			3,983,374	9,573	2,385,726	(24,057)	24,057	1,694,859	(106,784)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													27,798
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest										$ 4,291				4,291
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		(23,013)	27,076											(4,063)	[1]
Beginning Balance at Mar. 31, 2017			4,011,172
Stock options exercised				32	9,304
Deferred compensation employee stock trust				1	285
Stock-based compensation				89	38,994						$ 0
Employee tax withholdings by settlement of net share transactions		(13,051)		(34)	(13,017)
Shares repurchased and retired		(179,649)		(472)	(179,177)
APIC reclassified for MEP vesting					0
Shares Issued to plans						(286)	286
Distributions and forfeitures						2,617	(2,617)
Net Income Attributable to Legg Mason, Inc.			126,584					126,584
Dividends declared								(54,177)
RE Reclassified for MEP Amortization		$ 2,295
Excess Tax Benefit from Share-based Compensation, Financing Activities								24,327
Reclassification from Retained Earnings to Noncontrolling Interests												$ 0	$ (2,295)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax									240
Foreign currency translation adjustment			32,113						32,113
Ending Balance at Sep. 30, 2017			3,994,197
Stockholders' Equity Attributable to Parent			$ 3,966,171	$ 9,189	$ 2,242,115	$ (21,726)	$ 21,726	$ 1,789,298	$ (74,431)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													$ 28,026

[1] (2) Related to Royce management equity plan.