Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
OPERATING REVENUES
Revenues	$ 2,886,902	$ 2,660,844	$ 2,819,106
OPERATING EXPENSES
Compensation and benefits	1,374,334	1,172,645	1,208,214
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)		371,000
OTHER NON-OPERATING INCOME (EXPENSE)
Payments of Debt Extinguishment Costs			107,074
Net Income (Loss) Attributable to Noncontrolling Interest	59,447	(7,878)	5,629
Consolidated Legg Mason, Inc.
OPERATING REVENUES
Distribution and service fees	366,677	381,486	361,188
Other	4,653	2,237	5,694
Revenues	2,886,902	2,660,844	2,819,106
OPERATING EXPENSES
Compensation and benefits	1,401,648	1,204,817	1,232,770
Distribution and servicing	499,125	545,710	594,788
Communications and technology	208,885	197,857	182,438
Occupancy	113,714	122,610	109,708
Amortization of intangible assets	26,190	4,979	2,625
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	35,000	371,000	0
Other	180,097	163,040	198,558
Total Operating Expenses	2,464,659	2,610,013	2,320,887
OPERATING INCOME	422,243	50,831	498,219
OTHER NON-OPERATING INCOME (EXPENSE)
Interest income	6,815	5,634	7,440
Interest expense	(113,173)	(48,463)	(58,274)
Other income (expense), net	41,664	(25,977)	(85,280)
Total Other Non-Operating Income (Expense)	(51,365)	(76,049)	(130,226)
INCOME BEFORE INCOME TAX PROVISION	370,878	(25,218)	367,993
Income tax provision	84,175	7,692	125,284
NET INCOME	286,703	(32,910)	242,709
Net Income (Loss) Attributable to Noncontrolling Interest	59,447	(7,878)	5,629
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 227,256	$ (25,032)	$ 237,080
NET INCOME PER SHARE ATTRIBUTABLE TO LEGG MASON, INC. COMMON SHAREHOLDERS
Basic (in dollars per share)	$ 2.19	$ (0.25)	$ 2.06
Diluted (in dollars per share)	$ 2.18	$ (0.25)	$ 2.04
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 227,256	$ (25,032)	$ 237,080
Consolidated Investment Vehicles [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
Other income (expense), net	13,329	(7,243)	5,888
Separate Accounts [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	925,250	826,080	824,211
Mutual Funds [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	1,482,045	1,409,059	1,544,494
Performance Fees [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	108,277	41,982	83,519
Restructuring Charges [Member] | Consolidated Legg Mason, Inc.
OPERATING EXPENSES
Compensation and benefits	27,314	32,172	24,556
5.5% Senior Notes [Member] | Consolidated Legg Mason, Inc.
OTHER NON-OPERATING INCOME (EXPENSE)
Payments of Debt Extinguishment Costs	$ 0	$ 0	$ 107,074