Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	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]	Management Equity Plan [Member] Royce [Member]
Stockholders' Equity Attributable to Noncontrolling Interest at Mar. 31, 2017									$ 27,798
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest								$ 6,387	6,387
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders	$ (43,477)								(6,524)
Stock options exercised		$ 37	$ 10,593
Deferred compensation employee stock trust		1	419
Stock-based compensation		94	54,334
Employee tax withholdings by settlement of net share transactions		35	13,039
Shares repurchased and retired		(1,221)	(477,918)
Adjustments to Additional Paid in Capital, Fair Value			(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)
Reclassification from Retained Earnings to Noncontrolling Interests						(3,071)
Excess Tax Benefit from Share-based Compensation, Financing Activities						24,327
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	363
Foreign currency translation adjustment	32,303
Ending Balance at Dec. 31, 2017	3,833,492
Employee Tax Withholdings Related to net share transaction	(13,074)
Stockholders' Equity Attributable to Parent at Dec. 31, 2017	3,805,831	8,449	1,960,021	(21,860)	21,860	1,911,479	$ (74,118)
Stockholders' Equity Attributable to Noncontrolling Interest at Dec. 31, 2017									27,661
Common Stock Issued, Employee Stock Trust	21,996
Stockholders' Equity Attributable to Parent at Mar. 31, 2018	3,824,405
Stockholders' Equity Attributable to Noncontrolling Interest at Mar. 31, 2018									27,731
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest								$ 6,583	6,583
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders	32,358								(6,820)
Beginning Balance at Mar. 31, 2018	3,852,136
Stock options exercised		19	5,814
Deferred compensation employee stock trust		1	468
Stock-based compensation		112	49,195
Employee tax withholdings by settlement of net share transactions		40	15,507
Shares repurchased and retired		0	0
Adjustments to Additional Paid in Capital, Fair Value			12,251
Shares Issued to plans				(469)	469
Distributions and forfeitures				1,082	(1,082)
Net Income Attributable to Legg Mason, Inc.	(77,995)					(77,995)
Dividends declared						(90,345)
Reclassification from Retained Earnings to Noncontrolling Interests						(14,868)
Excess Tax Benefit from Share-based Compensation, Financing Activities						0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	1,447
Foreign currency translation adjustment	(82,032)
Ending Balance at Dec. 31, 2018	3,652,683
Employee Tax Withholdings Related to net share transaction	(15,547)
Stockholders' Equity Attributable to Parent at Dec. 31, 2018	3,625,189	$ 8,553	$ 2,028,586	$ (21,383)	$ 21,383	1,723,817	$ (135,767)
Stockholders' Equity Attributable to Noncontrolling Interest at Dec. 31, 2018									$ 27,494
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 12,263
Common Stock Issued, Employee Stock Trust	$ 21,383