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] Royce [Member]
Stockholders' Equity Attributable to Parent				$ 11,717	$ 3,148,396	$ (29,922)	$ 29,922	$ 1,526,662	$ 37,949
Stockholders' Equity Attributable to Noncontrolling Interest														$ 0
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest		$ 0												0
Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense														0
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders														0
Stock options exercised				71	21,994
Deferred compensation employee stock trust				5	2,218
Stock-based compensation				94	54,935							$ 0
Adjustments to Additional Paid in Capital, Other					0
Employee tax withholdings by settlement of net share transactions				(47)	(22,067)
Shares repurchased and retired		(356,522)		(693)	(355,829)
APIC reclassified for MEP vesting					(5,206)
Shares Issued to plans						(2,223)	2,223
Distributions and forfeitures						2,575	(2,575)
Net Income Attributable to Legg Mason, Inc.			$ 237,080					237,080
Dividends declared								(73,687)
RE Reclassified for MEP Amortization		5,206						0
Reclassification from Retained Earnings to Noncontrolling Interests													$ 0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax			(9,595)						(9,595)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax			(114)						(114)
Foreign currency translation adjustment			(88,982)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax			0						0
Ending Balance at Mar. 31, 2015			4,484,901
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)		638
Stockholders' Equity Attributable to Parent			4,484,901	11,147	2,844,441	(29,570)	29,570	1,690,055	(60,742)
Stockholders' Equity Attributable to Noncontrolling Interest										$ 0	[1]			0
Net Income (Loss) Attributable to Nonredeemable Noncontrolling Interest										802				802
Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense														21,400
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders														0
Stock options exercised				33	9,482
Deferred compensation employee stock trust				2	505
Stock-based compensation				14	65,373							0
Adjustments to Additional Paid in Capital, Other					9,173
Employee tax withholdings by settlement of net share transactions				(41)	(21,596)
Shares repurchased and retired		(209,632)		(454)	(209,178)
APIC reclassified for MEP vesting					(5,087)
Shares Issued to plans						(507)	507
Distributions and forfeitures						3,814	(3,814)
Net Income Attributable to Legg Mason, Inc.			(25,032)					(25,032)
Dividends declared								(87,818)
RE Reclassified for MEP Amortization		6,050						(963)
Reclassification from Retained Earnings to Noncontrolling Interests													0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax			2,774						2,774
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax			0						0
Foreign currency translation adjustment		(59,672)	(8,525)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax			0						0
Ending Balance at Mar. 31, 2016			4,235,765
Stockholders' Equity Attributable to Parent			4,213,563	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										$ 7,397				7,397
Deferred Compensation Arrangement with Individual, Allocated Share-based Compensation Expense														4,600
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		(29,461)												(6,401)	[1]
Stock options exercised				32	8,958
Deferred compensation employee stock trust				2	514
Stock-based compensation				45	71,494							$ 11,121
Adjustments to Additional Paid in Capital, Other					0
Employee tax withholdings by settlement of net share transactions				(37)	(12,102)
Shares repurchased and retired		(381,672)		(1,170)	(380,502)
APIC reclassified for MEP vesting					(6,870)
Shares Issued to plans						(516)	516
Distributions and forfeitures						2,722	(2,722)
Net Income Attributable to Legg Mason, Inc.			227,256					227,256
Dividends declared								(90,578)
RE Reclassified for MEP Amortization		9,335						(2,561)
Reclassification from Retained Earnings to Noncontrolling Interests													$ (15,500)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax			(9,860)						(9,860)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax			0						0
Foreign currency translation adjustment		$ (90,103)	(32,924)						(32,924)
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 Mar. 31, 2017			4,011,172
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

[1] Related to Royce management equity plan.