Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
OPERATING REVENUES
Revenues	$ 3,140,322	$ 2,886,902	$ 2,660,844
OPERATING EXPENSES
Compensation and benefits	1,503,000	1,374,334	1,172,645
OTHER NON-OPERATING INCOME (EXPENSE)
Income tax provision	(102,510)	84,175	7,692
Net Income (Loss) Attributable to Noncontrolling Interest	$ 51,275	$ 59,447	$ (7,878)
NET INCOME PER SHARE ATTRIBUTABLE TO LEGG MASON, INC. COMMON SHAREHOLDERS
Common Stock, Dividends, Per Share, Declared	$ 1.12	$ 0.88	$ 0.80
EnTrust Capital [Member]
OPERATING REVENUES
Revenues		$ 115,327
Consolidated Legg Mason, Inc.
OPERATING REVENUES
Distribution and service fees	$ 321,936	366,677	$ 381,486
Other	4,972	4,653	2,237
Revenues	3,140,322	2,886,902	2,660,844
OPERATING EXPENSES
Compensation and benefits	1,508,798	1,401,648	1,204,817
Distribution and servicing	489,331	499,125	545,710
Communications and technology	212,798	208,885	197,857
Occupancy	100,760	113,714	122,610
Amortization of intangible assets	24,604	26,190	4,979
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	229,000	35,000	371,000
Contingent consideration fair value adjustments	(31,329)	(39,500)	(33,375)
Other	282,359	219,597	196,415
Total Operating Expenses	2,816,321	2,464,659	2,610,013
OPERATING INCOME	324,001	422,243	50,831
OTHER NON-OPERATING INCOME (EXPENSE)
Interest income	7,106	6,815	5,634
Interest expense	117,872	113,173	(48,463)
Other income (expense), net	10,824	41,664	(25,977)
Total Other Non-Operating Income (Expense)	(90,161)	(51,365)	(76,049)
INCOME BEFORE INCOME TAX PROVISION	233,840	370,878	(25,218)
Income tax provision	(102,510)	84,175	7,692
NET INCOME	336,350	286,703	(32,910)
Net Income (Loss) Attributable to Noncontrolling Interest	51,275	59,447	(7,878)
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 285,075	$ 227,256	$ (25,032)
NET INCOME PER SHARE ATTRIBUTABLE TO LEGG MASON, INC. COMMON SHAREHOLDERS
Basic (in dollars per share)	$ 3.03	$ 2.19	$ (0.25)
Diluted (in dollars per share)	$ 3.01	$ 2.18	$ (0.25)
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 285,075	$ 227,256	$ (25,032)
Consolidated Investment Vehicles [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
Other income (expense), net	9,781	13,329	(7,243)
Separate Accounts [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	1,020,790	925,250	826,080
Mutual Funds [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	1,564,839	1,482,045	1,409,059
Performance Fees [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Investment Advisory Fees	227,785	108,277	41,982
Restructuring Charges [Member] | Consolidated Legg Mason, Inc.
OPERATING EXPENSES
Compensation and benefits	$ 5,798	$ 27,314	$ 32,172