Variable Interest Entities and Consolidation of Investment Vehicles (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Balance Before Consolidation of CIVs
Total operating revenues	$ 674,506	$ 627,731	$ 1,946,667	$ 2,016,413
Total operating expenses	1,307,124	567,550	2,421,982	1,746,967
Operating income (loss)	(632,618)	60,181	(475,315)	269,446
Total other non-operating income (expense)	(1,385)	(19,357)	(75,651)	(85,586)
Income (loss) before income tax provision (benefit)	(634,003)	40,824	(550,966)	183,860
Income tax provision (benefit)	(180,214)	12,607	(168,814)	38,868
Net income (loss)	(453,789)	28,217	(382,152)	144,992
Less: Net income (loss) attributable to noncontrolling interests	81	85	379	244
Net income (loss) attributable to Legg Mason, Inc.	(453,870)	28,132	(382,531)	144,748
CIVs
Total operating revenues	0	0	0	0
Total operating expenses	703	859	2,196	2,906
Operating income (loss)	(703)	(859)	(2,196)	(2,906)
Total other non-operating income (expense)	(3,458)	7,424	(6,089)	15,607
Income (loss) before income tax provision (benefit)	(4,161)	6,565	(8,285)	12,701
Income tax provision (benefit)	0	0	0	0
Net income (loss)	(4,161)	6,565	(8,285)	12,701
Less: Net income (loss) attributable to noncontrolling interests	0	0	0	0
Net income (loss) attributable to Legg Mason, Inc.	(4,161)	6,565	(8,285)	12,701
Eliminations
Total operating revenues	(606)	(753)	(1,780)	(2,430)
Total operating expenses	(604)	(754)	(1,779)	(2,431)
Operating income (loss)	(2)	1	(1)	1
Total other non-operating income (expense)	(598)	358	(1)	(3,472)
Income (loss) before income tax provision (benefit)	(600)	359	(2)	(3,471)
Income tax provision (benefit)	0	0	0	0
Net income (loss)	(600)	359	(2)	(3,471)
Less: Net income (loss) attributable to noncontrolling interests	(4,761)	6,924	(8,287)	9,230
Net income (loss) attributable to Legg Mason, Inc.	4,161	(6,565)	8,285	(12,701)
Consolidated Totals
Total operating revenues	673,900	626,978	1,944,887	2,013,983
Total operating expenses	1,307,223	567,655	2,422,399	1,747,442
Operating income (loss)	(633,323)	59,323	(477,512)	266,541
Total other non-operating income (expense)	(5,441)	(11,575)	(81,741)	(73,451)
Income (loss) before income tax provision (benefit)	(638,764)	47,748	(559,253)	193,090
Income tax provision (benefit)	(180,214)	12,607	(168,814)	38,868
Net income (loss)	(458,550)	35,141	(390,439)	154,222
Less: Net income (loss) attributable to noncontrolling interests	(4,680)	7,009	(7,908)	9,474
Net income (loss) attributable to Legg Mason, Inc.	$ (453,870)	$ 28,132	$ (382,531)	$ 144,748