Earnings Per Share (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2011 Consolidated Legg Mason Group [Member]	Sep. 30, 2011 Consolidated Legg Mason Group [Member]	Sep. 30, 2011 Consolidated Legg Mason Group [Member] Senior Notes from Equity Units [Member]	Jun. 30, 2011 Consolidated Legg Mason Group [Member] Senior Notes from Equity Units [Member]	Sep. 30, 2011 Consolidated Legg Mason Group [Member] Senior Notes from Equity Units [Member]	Sep. 30, 2011 Stock options	Sep. 30, 2010 Stock options	Sep. 30, 2011 Stock options	Sep. 30, 2010 Stock options
Additional earnings per share disclosures
Debt instrument interest rate stated percentage (as a percent)	2.50%	2.50%				5.60%
Common stock issued through Equity Units						1,830
Incremental Common Shares Attributable to Conversion of Debt Securities					1,830		930
Common stock purchased and retired (in shares)	7,564	13,597
Common stock purchased and retired in the open market			$ 200,147	$ 400,266
Repurchased and Retired Shares, Excluded from Weighted Average Shares Outstanding Number	9,736	5,835
Number of antidilutive shares excluded from EPS calculation								5,379	5,504	7,144	5,407
Stock Issued During Period, Shares, Restricted Stock Award, Gross	56	1,303
Restricted Stock Included In Weighted-Average Shares Outstanding	1,286	960