Earnings Per Share (Details 3) (EPS calculation, USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended			9 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2012		Dec. 31, 2011
EPS calculation
Potential common shares:
Weighted-average basic shares outstanding (in shares)	131,534	[1]	140,053	134,770	[1]	144,363
Employee stock options (in shares)	0	[1]	29	0	[1]	65
Weighted-average diluted shares (in shares)	131,534	[1]	140,082	134,770	[1]	144,428
Net Income (Loss)	$ (458,550)	[1]	$ 35,141	$ (390,439)	[1]	$ 154,222
Less: Net income (loss) attributable to noncontrolling interests	(4,680)	[1]	7,009	(7,908)	[1]	9,474
Net income (loss) attributable to Legg Mason, Inc.	$ (453,870)	[1]	$ 28,132	$ (382,531)	[1]	$ 144,748
Basic (in dollars per share)	$ (3.45)		$ 0.20	$ (2.84)		$ 1.00
Diluted (in dollars per share)	$ (3.45)	[1]	$ 0.20	$ (2.84)	[1]	$ 1.00

[1] Diluted shares are the same as basic shares for periods with a net loss.