Variable Interest Entities and Consolidation of Investment Vehicles (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Balance Before Consolidation of CIVs
Total operating revenues	$ 2,615,047	$ 2,665,668	$ 2,788,450
Total operating expenses	3,046,587	2,323,213	2,396,938
Operating income (loss)	(431,540)	342,455	391,512
Total other non-operating income (expense)	(72,177)	(49,236)	(17,931)
Income (loss) before income tax provision (benefit)	(503,717)	293,219	373,581
Income tax provision (benefit)	(150,859)	72,052	119,434
Net income (loss)	(352,858)	221,167	254,147
Less: Net income (loss) attributable to noncontrolling interests	469	350	224
Net income (loss) attributable to Legg Mason, Inc.	(353,327)	220,817	253,923
CIVs
Total operating revenues	0	0	0
Total operating expenses	2,965	3,709	4,704
Operating income (loss)	(2,965)	(3,709)	(4,704)
Total other non-operating income (expense)	(2,864)	18,336	1,704
Income (loss) before income tax provision (benefit)	(5,829)	14,627	(3,000)
Income tax provision (benefit)	0	0	0
Net income (loss)	(5,829)	14,627	(3,000)
Less: Net income (loss) attributable to noncontrolling interests	0	0	0
Net income (loss) attributable to Legg Mason, Inc.	(5,829)	14,627	(3,000)
Eliminations
Total operating revenues	(2,397)	(3,094)	(4,133)
Total operating expenses	(2,403)	(3,101)	(4,133)
Operating income (loss)	6	7	0
Total other non-operating income (expense)	(1,067)	(4,770)	(5,384)
Income (loss) before income tax provision (benefit)	(1,061)	(4,763)	(5,384)
Income tax provision (benefit)	0	0	0
Net income (loss)	(1,061)	(4,763)	(5,384)
Less: Net income (loss) attributable to noncontrolling interests	(6,890)	9,864	(8,384)
Net income (loss) attributable to Legg Mason, Inc.	5,829	(14,627)	3,000
Consolidated Legg Mason
Total operating revenues	2,612,650	2,662,574	2,784,317
Total operating expenses	3,047,149	2,323,821	2,397,509
Operating income (loss)	(434,499)	338,753	386,808
Total other non-operating income (expense)	(76,108)	(35,670)	(21,611)
Income (loss) before income tax provision (benefit)	(510,607)	303,083	365,197
Income tax provision (benefit)	(150,859)	72,052	119,434
Net income (loss)	(359,748)	231,031	245,763
Less: Net income (loss) attributable to noncontrolling interests	(6,421)	10,214	(8,160)
Net income (loss) attributable to Legg Mason, Inc.	$ (353,327)	$ 220,817	$ 253,923