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										$ 5,732				5,732
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		$ (18,797)	$ 23,351											(4,555)	[1]
Stock options exercised				14	3,906
Deferred compensation employee stock trust				1	385
Stock-based compensation				41	60,567						$ 11,121
Employee tax withholdings by settlement of net share transactions		(11,845)		(36)	(11,809)
Shares repurchased and retired		(291,674)		(912)	(290,762)
APIC reclassified for MEP vesting					(3,632)
Shares Issued to plans						(386)	386
Distributions and forfeitures						1,663	(1,663)
Net Income Attributable to Legg Mason, Inc.			151,332					151,332
Dividends declared								(68,377)
RE Reclassified for MEP Amortization		(6,261)						(2,629)
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									(16,483)
Foreign currency translation adjustment			(55,797)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax		2,493	(2,493)
Ending Balance at Dec. 31, 2016			4,000,865
Stockholders' Equity Attributable to Parent			3,977,486	9,809	2,462,889	(24,986)	24,986	1,641,068	(136,280)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													23,379
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										$ 6,387				6,387
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		(36,953)	43,477											(6,524)	[1]
Beginning Balance at Mar. 31, 2017			4,011,172
Stock options exercised				37	10,593
Deferred compensation employee stock trust				1	419
Stock-based compensation				94	54,334						$ 0
Employee tax withholdings by settlement of net share transactions		(13,074)		(35)	(13,039)
Shares repurchased and retired		(253,649)		(1,221)	(477,918)
APIC reclassified for MEP vesting					(94)
Shares Issued to plans						(420)	420
Distributions and forfeitures						2,617	(2,617)
Net Income Attributable to Legg Mason, Inc.			275,806					275,806
Dividends declared								(80,442)
RE Reclassified for MEP Amortization		$ (3,165)
Excess Tax Benefit from Share-based Compensation, Financing Activities								24,327
Reclassification from Retained Earnings to Noncontrolling Interests												$ 0	$ (3,071)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax									363
Foreign currency translation adjustment			32,303						32,303
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax			0
Ending Balance at Dec. 31, 2017			3,833,492
Stockholders' Equity Attributable to Parent			$ 3,805,831	$ 8,449	$ 1,960,021	$ (21,860)	$ 21,860	$ 1,911,479	$ (74,118)
Stockholders' Equity Attributable to Noncontrolling Interest	[1]													$ 27,661

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