Variable Interest Entities and Consolidation of Investment Vehicles (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2011 Before Consolidation [Member]	Mar. 31, 2011 Before Consolidation [Member]	Dec. 31, 2011 CIVs	Mar. 31, 2011 CIVs		Dec. 31, 2011 Eliminations	Mar. 31, 2011 Eliminations	Dec. 31, 2011 Consolidated Legg Mason Group [Member]	Mar. 31, 2011 Consolidated Legg Mason Group [Member]	Dec. 31, 2010 Consolidated Legg Mason Group [Member]	Mar. 31, 2010 Consolidated Legg Mason Group [Member]
Variable Interest Entity [Line Items]
Controlling financial interest in VRE redeemed	$ 20,814
Variable Interest Entities and Consolidation of Investment Vehicles
Number of sponsored investment fund VIEs for which Legg Mason is the primary beneficiary	1
Number of sponsored investment fund VREs for which Legg Mason has a controlling financial interest	1
Number of CLOs reviewed to determine whether Legg Mason is the primary beneficiary.	2
Number of CLOs for which Legg Mason is the primary beneficiary	1
Investment in consolidated sponsored investment funds	43,145	53,708
Current assets			2,204,535	2,378,226	70,678	122,963	[1]	(43,667)	(54,633)	2,231,546	2,446,556
Non-current assets			5,847,514	5,946,737	292,002	314,463	[1]	0	0	6,139,516	6,261,200
Total Assets			8,052,049	8,324,963	362,680	437,426	[1]	(43,667)	(54,633)	8,371,062	8,707,756
Current liabilities			891,674	914,803	9,785	55,094	[1]	(521)	(925)	900,938	968,972
Long-term debt of CIVs			0	0	265,478	278,320	[1]	0	0	265,478	278,320
Other non-current liabilities			1,574,860	1,649,815	3,998	3,553	[1]	0	0	1,578,858	1,653,368
Total Liabilities			2,466,534	2,564,618	279,261	336,967	[1]	(521)	(925)	2,745,274	2,900,660
Redeemable Noncontrolling Interests			911	976	0	0	[1]	27,905	35,736	28,816	36,712	32,294	29,577
Total stockholders' equity			5,584,604	5,759,369	83,419	100,459	[1]	(71,051)	(89,444)	5,596,972	5,770,384	5,762,256
Total Liabilities and Stockholders' Equity			$ 8,052,049	$ 8,324,963	$ 362,680	$ 437,426	[1]	$ (43,667)	$ (54,633)	$ 8,371,062	$ 8,707,756

[1] Certain immaterial non-current liabilities of CIVs are included in Other liabilities on the Consolidated Balance Sheets.