Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
OPERATING REVENUES
Total Operating Revenues	$ 758,427	$ 768,338	$ 1,506,332	$ 1,562,180
OPERATING EXPENSES
OPERATING INCOME
OTHER NON-OPERATING INCOME (EXPENSE)
INCOME BEFORE INCOME TAX PROVISION
NET INCOME
Net Income (Loss) Attributable to Noncontrolling Interest	8,270	11,960	20,545	24,577
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.
Consolidated Legg Mason, Inc.
OPERATING REVENUES
Other	1,989	686	3,209	1,811
Total Operating Revenues	758,427	768,338	1,506,332	1,562,180
OPERATING EXPENSES
Compensation and benefits	364,885	367,951	726,453	781,258
Distribution and servicing	114,525	123,634	231,117	245,983
Communications and technology	57,489	51,299	114,229	101,602
Occupancy	27,352	25,171	52,256	49,579
Amortization of intangible assets	6,102	6,082	12,282	12,421
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	0	0	0	34,000
Contingent consideration fair value adjustments	145	0	571	(16,550)
Other	52,201	49,782	108,020	102,263
Total Operating Expenses	622,699	623,919	1,244,928	1,310,556
OPERATING INCOME	135,728	144,419	261,404	251,624
OTHER NON-OPERATING INCOME (EXPENSE)
Interest income	2,420	1,572	4,866	3,040
Interest expense	(29,860)	(29,077)	(59,777)	(58,343)
Other income (expense), net	6,627	7,289	13,879	18,677
Total Other Non-Operating Income (Expense)	(24,811)	(18,122)	(41,447)	(33,535)
INCOME BEFORE INCOME TAX PROVISION	110,917	126,297	219,957	218,089
Income tax provision	29,844	38,673	60,519	66,928
NET INCOME	81,073	87,624	159,438	151,161
Net Income (Loss) Attributable to Noncontrolling Interest	8,270	11,960	20,545	24,577
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 72,803	$ 75,664	$ 138,893	$ 126,584
NET INCOME PER SHARE ATTRIBUTABLE TO LEGG MASON, INC. COMMON SHAREHOLDERS
Basic (in dollars per share)	$ 0.82	$ 0.78	$ 1.57	$ 1.30
Diluted (in dollars per share)	0.82	0.78	1.57	1.29
Common Stock, Dividends, Per Share, Declared	$ 0.34	$ 0.28	$ 0.68	$ 0.56
Consolidated Entity Excluding Consolidated Investment Vehicles Before Eliminations [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
NET INCOME ATTRIBUTABLE TO LEGG MASON, INC.	$ 72,803	$ 75,664	$ 138,893	$ 126,584
Consolidated Investment Vehicles [Member]
OTHER NON-OPERATING INCOME (EXPENSE)
Other income (expense), net	(3,998)	2,094	(415)	3,091
Separate Accounts [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Revenue from Contract with Customer, Excluding Assessed Tax	261,567	253,128	521,462	503,174
Mutual Funds [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Revenue from Contract with Customer, Excluding Assessed Tax	383,923	393,035	767,487	775,263
Performance Fees [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Revenue from Contract with Customer, Excluding Assessed Tax	31,874	40,821	55,910	122,358
Distribution and Shareholder Service [Member] | Consolidated Legg Mason, Inc.
OPERATING REVENUES
Revenue from Contract with Customer, Excluding Assessed Tax	$ 79,074	$ 80,668	$ 158,264	$ 159,574