VIE Consolidated Income Statements Details (Details 2) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	$ 19,324		$ 13,088		$ 43,901	$ 45,067
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	793,373	[1]	715,601		2,355,620	2,164,162
Total Operating Expenses	819,984	[1]	604,075		2,130,412	1,851,199
OPERATING INCOME	(26,611)	[1]	111,526		225,208	312,963
Total Other Non-Operating Income (Expense)	(19,970)	[1]	(20,545)		(55,892)	(53,063)
INCOME BEFORE INCOME TAX PROVISION	(46,581)	[1]	90,981		169,316	259,900
Income tax provision	(209,396)	[1]	26,441		(142,468)	71,654
NET INCOME	162,815	[1]	64,540		311,784	188,246
Net Income (Loss) Attributable to Noncontrolling Interest	13,593	[1]	13,101		35,978	36,914
Net Income Attributable to Legg Mason, Inc.	149,222	[1]	51,439		275,806	151,332
Variable Interest Entity, Primary Beneficiary [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	0	[1]	0	[1]	0	0
Total Operating Expenses	555	[1]	383		749	606
OPERATING INCOME	(555)	[1]	(383)		(749)	(606)
Total Other Non-Operating Income (Expense)	7,940	[1]	793		11,310	10,444
INCOME BEFORE INCOME TAX PROVISION	7,385	[1]	410		10,561	9,838
Income tax provision	0	[1]	0	[1]	0	0
NET INCOME	7,385	[1]	410		10,561	9,838
Net Income (Loss) Attributable to Noncontrolling Interest	(285)	[1]	(4)		(5)	552
Net Income Attributable to Legg Mason, Inc.	7,670	[1]	414		10,566	9,286
Consolidated Legg Mason, Inc.
Condensed Income Statements, Captions [Line Items]
Revenues	793,090		715,241		2,355,270	2,163,776
Total Operating Expenses	820,423		604,075		2,130,979	1,851,422
OPERATING INCOME	(27,333)		111,166		224,291	312,354
Total Other Non-Operating Income (Expense)	(13,517)		(20,198)		(47,052)	(44,301)
INCOME BEFORE INCOME TAX PROVISION	(40,850)		90,968		177,239	268,053
Income tax provision	(209,396)		26,441		(142,468)	71,654
NET INCOME	168,546		64,527		319,707	196,399
Net Income (Loss) Attributable to Noncontrolling Interest	19,324		13,088		43,901	45,067
Net Income Attributable to Legg Mason, Inc.	149,222		51,439		275,806	151,332
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Revenues	(283)		(360)		(350)	(386)
Total Operating Expenses	(116)		(383)		(182)	(383)
OPERATING INCOME	(167)		23		(168)	(3)
Total Other Non-Operating Income (Expense)	(1,487)		(446)		(2,470)	(1,682)
INCOME BEFORE INCOME TAX PROVISION	(1,654)		(423)		(2,638)	(1,685)
Income tax provision	0		0		0	0
NET INCOME	(1,654)		(423)		(2,638)	(1,685)
Net Income (Loss) Attributable to Noncontrolling Interest	6,016		(9)		7,928	7,601
Net Income Attributable to Legg Mason, Inc.	$ (7,670)		$ (414)		$ (10,566)	$ (9,286)

[1] Three Months Ended December 31, 2017 December 31, 2016 Balance BeforeConsolidation of CIVs and Other(1) CIVs and Other(1) Eliminations Consolidated Totals Balance BeforeConsolidation of CIVs and Other(1) CIVs and Other(1) Eliminations Consolidated TotalsTotal Operating Revenues $793,373 $— $(283) $793,090 $715,601 $— $(360) $715,241Total Operating Expenses 819,984 555 (116) 820,423 604,075 383 (383) 604,075Operating Income (Loss) (26,611) (555) (167) (27,333) 111,526 (383) 23 111,166Total Non-Operating Income (Expense) (19,970) 7,940 (1,487) (13,517) (20,545) 793 (446) (20,198)Income (Loss) Before Income Tax Provision (Benefit) (46,581) 7,385 (1,654) (40,850) 90,981 410 (423) 90,968Income tax provision (benefit) (209,396) — — (209,396) 26,441 — — 26,441Net Income 162,815 7,385 (1,654) 168,546 64,540 410 (423) 64,527Less: Net income (loss) attributable to noncontrolling interests 13,593 (285) 6,016 19,324 13,101 (4) (9) 13,088Net Income Attributable to Legg Mason, Inc. $149,222 $7,670 $(7,670) $149,222 $51,439 $414 $(414) $51,439(1)Other represents consolidated sponsored investment products (VREs) that are not designated as CIVs.