Variable Interest Entities and Consolidation of Investment Vehicles (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Mar. 31, 2012 Before Consolidation	Mar. 31, 2011 Before Consolidation	Mar. 31, 2012 CIVs	Mar. 31, 2011 CIVs	Mar. 31, 2012 Eliminations	Mar. 31, 2011 Eliminations	Mar. 31, 2012 Legg Mason, Inc.	Mar. 31, 2011 Legg Mason, Inc.	Mar. 31, 2010 Legg Mason, Inc.	Mar. 31, 2009 Legg Mason, Inc.
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	38,919		53,708
Current assets				2,439,162	2,378,226	58,040	122,963	(39,408)	(54,633)	2,457,794	2,446,556
Non-current assets				5,801,680	5,946,737	296,273	314,463	0	0	6,097,953	6,261,200
Total assets				8,240,842	8,324,963	354,313	437,426	(39,408)	(54,633)	8,555,747	8,707,756
Current liabilities				971,804	914,803	4,467	55,094	(489)	(925)	975,782	968,972
Long-term debt of CIVs				0	0	271,707	278,320	0	0	271,707	278,320
Other non-current liabilities				1,603,064	1,649,815	3,872	3,553	0	0	1,606,936	1,653,368
Total liabilities				2,574,868	2,564,618	280,046	336,967	(489)	(925)	2,854,425	2,900,660
Redeemable noncontrolling interests				996	976	0	0	23,035	35,736	24,031	36,712	29,577	31,020
Total stockholders' equity				5,664,978	5,759,369	74,267	100,459	(61,954)	(89,444)	5,677,291	5,770,384	5,841,724
Total liabilities and stockholders' equity				$ 8,240,842	$ 8,324,963	$ 354,313	$ 437,426	$ (39,408)	$ (54,633)	$ 8,555,747	$ 8,707,756