VIE Consolidated Income Statements Details (Details 2) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Condensed Income Statements, Captions [Line Items]
Total Operating Revenues			$ 2,903,259	$ 3,140,322	$ 2,886,902
OPERATING INCOME
INCOME BEFORE INCOME TAX PROVISION
Income tax provision			20,561	(102,510)	84,175
NET INCOME				336,350
Net Income (Loss) Attributable to Noncontrolling Interest			36,442	51,275	59,447
Net Income Attributable to Legg Mason, Inc.
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Condensed Income Statements, Captions [Line Items]
Total Operating Revenues	$ 2,903,858	$ 2,355,620	2,903,858	3,140,900	2,887,431
Total Operating Expenses	2,799,168	2,130,412	2,799,168	2,816,022	2,464,369
OPERATING INCOME	104,690	225,208	104,690	324,878	423,062
Total Other Non-Operating Income (Expense)	(76,971)	(55,892)	(76,971)	(97,694)	(63,636)
INCOME BEFORE INCOME TAX PROVISION	27,719	169,316	27,719	227,184	359,426
Income tax provision	20,561	(142,468)	20,561	(102,510)	84,175
NET INCOME	7,158	311,784	7,158	329,694	275,251
Net Income (Loss) Attributable to Noncontrolling Interest	35,666	35,978	35,666	44,619	47,995
Net Income Attributable to Legg Mason, Inc.	(28,508)	275,806	(28,508)	285,075	227,256
Variable Interest Entity, Primary Beneficiary [Member]
Condensed Income Statements, Captions [Line Items]
Total Operating Revenues	0	0	0	0	0
Total Operating Expenses	0	749	1,679	927	816
OPERATING INCOME	0	(749)	(1,679)	(927)	(816)
Total Other Non-Operating Income (Expense)	0	11,310	(2,381)	10,046	15,602
INCOME BEFORE INCOME TAX PROVISION	0	10,561	(4,060)	9,119	14,786
Income tax provision	0	0	0	0	0
NET INCOME	0	10,561	(4,060)	9,119	14,786
Net Income (Loss) Attributable to Noncontrolling Interest	0	(5)	(1,816)	265	2,274
Net Income Attributable to Legg Mason, Inc.	0	10,566	(2,244)	8,854	12,512
Consolidated Legg Mason, Inc.
Condensed Income Statements, Captions [Line Items]
Total Operating Revenues	2,903,858	2,355,270	2,903,259	3,140,322	2,886,902
Total Operating Expenses	2,799,168	2,130,979	2,800,157	2,816,321	2,464,659
OPERATING INCOME	104,690	224,291	103,102	324,001	422,243
Total Other Non-Operating Income (Expense)	(76,971)	(47,052)	(74,607)	(90,161)	(51,365)
INCOME BEFORE INCOME TAX PROVISION	27,719	177,239	28,495	233,840	370,878
Income tax provision	20,561	(142,468)	20,561	(102,510)	84,175
NET INCOME	7,158	319,707	7,934	336,350	286,703
Net Income (Loss) Attributable to Noncontrolling Interest	35,666	43,901	36,442	51,275	59,447
Net Income Attributable to Legg Mason, Inc.	(28,508)	275,806	(28,508)	285,075	227,256
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Total Operating Revenues	0	(350)	(599)	(578)	(529)
Total Operating Expenses	0	(182)	(690)	(628)	(526)
OPERATING INCOME	0	(168)	91	50	(3)
Total Other Non-Operating Income (Expense)	0	(2,470)	4,745	(2,513)	(3,331)
INCOME BEFORE INCOME TAX PROVISION	0	(2,638)	4,836	(2,463)	(3,334)
Income tax provision	0	0	0	0	0
NET INCOME	0	(2,638)	4,836	(2,463)	(3,334)
Net Income (Loss) Attributable to Noncontrolling Interest	0	7,928	2,592	6,391	9,178
Net Income Attributable to Legg Mason, Inc.	$ 0	$ (10,566)	$ 2,244	$ (8,854)	$ (12,512)