VIE Consolidated Income Statements Details (Details 2) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017	Sep. 30, 2016
Condensed Income Statements, Captions [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	$ 11,960		$ 20,091		$ 24,577	$ 31,979
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	768,361	[1]	748,384	[1]	1,562,247	1,448,561
Total Operating Expenses	623,814	[1]	620,613	[1]	1,310,428	1,247,124
OPERATING INCOME	144,547	[1]	127,771	[1]	251,819	201,437
Total Other Non-Operating Income (Expense)	(19,794)	[1]	(17,023)	[1]	(35,922)	(32,518)
INCOME BEFORE INCOME TAX PROVISION	124,753	[1]	110,748	[1]	215,897	168,919
Income tax provision	38,673	[1]	29,902	[1]	66,928	45,213
NET INCOME	86,080	[1]	80,846	[1]	148,969	123,706
Net Income (Loss) Attributable to Noncontrolling Interest	10,416	[1]	14,405	[1]	22,385	23,813
Net Income Attributable to Legg Mason, Inc.	75,664	[1]	66,441	[1]	126,584	99,893
Variable Interest Entity, Primary Beneficiary [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	0	[1]	0	[1]	0	(12)
Total Operating Expenses	127	[1]	124	[1]	195	223
OPERATING INCOME	(127)	[1]	(124)	[1]	(195)	(235)
Total Other Non-Operating Income (Expense)	2,131	[1]	7,103	[1]	3,371	9,651
INCOME BEFORE INCOME TAX PROVISION	2,004	[1]	6,979	[1]	3,176	9,416
Income tax provision	0	[1]	0	[1]	0	0
NET INCOME	2,004	[1]	6,979	[1]	3,176	9,416
Net Income (Loss) Attributable to Noncontrolling Interest	35	[1]	554	[1]	279	556
Net Income Attributable to Legg Mason, Inc.	1,969	[1]	6,425	[1]	2,897	8,860
Consolidated Legg Mason, Inc.
Condensed Income Statements, Captions [Line Items]
Revenues	768,338		748,370		1,562,180	1,448,535
Total Operating Expenses	623,919		620,737		1,310,556	1,247,347
OPERATING INCOME	144,419		127,633		251,624	201,188
Total Other Non-Operating Income (Expense)	(18,122)		(11,199)		(33,535)	(24,103)
INCOME BEFORE INCOME TAX PROVISION	126,297		116,434		218,089	177,085
Income tax provision	38,673		29,902		66,928	45,213
NET INCOME	87,624		86,532		151,161	131,872
Net Income (Loss) Attributable to Noncontrolling Interest	11,960		20,091		24,577	31,979
Net Income Attributable to Legg Mason, Inc.	75,664		66,441		126,584	99,893
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	(23)		(14)		(67)	(14)
Total Operating Expenses	(22)		0		(67)	0
OPERATING INCOME	(1)		(14)		0	(14)
Total Other Non-Operating Income (Expense)	(459)		(1,279)		(984)	(1,236)
INCOME BEFORE INCOME TAX PROVISION	(460)		(1,293)		(984)	(1,250)
Income tax provision	0		0		0	0
NET INCOME	(460)		(1,293)		(984)	(1,250)
Net Income (Loss) Attributable to Noncontrolling Interest	1,509		5,132		1,913	7,610
Net Income Attributable to Legg Mason, Inc.	$ (1,969)		$ (6,425)		$ (2,897)	$ (8,860)

[1] Other represents consolidated sponsored investment products (VREs) that are not designated as CIVs.