VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009	Mar. 31, 2008
VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES
Number of collateralized investment vehicles considered VIEs	2
Number of CLOs for which Legg Mason is the primary beneficiary	1
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 is the primary beneficiary	2
Investment in consolidated sponsored investment funds	$ 53,708	$ 61,864
Current assets	2,446,556	2,559,146
Non-current assets	6,261,200	6,063,486
Total Assets	8,707,756	8,622,632
Current liabilities	968,972	1,054,276
Long-term debt of CIVs	278,320
Other noncurrent liabilities	1,653,368	1,697,055
Total Liabilities	2,900,660	2,751,331
Redeemable noncontrolling interests	36,712	29,577	31,020	92
Total stockholders' equity	5,770,384	5,841,724	4,598,625
Total Liabilities and Stockholders' Equity	8,707,756	8,622,632
Balance Before Consolidation of CIVs
VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES
Current assets	2,378,226	2,541,880
Non-current assets	5,946,737	6,049,794
Total Assets	8,324,963	8,591,674
Current liabilities	914,803	1,053,893
Other noncurrent liabilities	1,649,815	1,697,055
Total Liabilities	2,564,618	2,750,948
Redeemable noncontrolling interests	976	667
Total stockholders' equity	5,759,369	5,840,059
Total Liabilities and Stockholders' Equity	8,324,963	8,591,674
CIVs
VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES
Current assets	122,963	79,692
Non-current assets	314,463	13,692
Total Assets	437,426	93,384
Current liabilities	55,094	961
Long-term debt of CIVs	278,320
Other noncurrent liabilities	3,553
Total Liabilities	336,967	961
Total stockholders' equity	100,459	92,423
Total Liabilities and Stockholders' Equity	437,426	93,384
Eliminations
VARIABLE INTEREST ENTITIES AND CONSOLIDATION OF INVESTMENT VEHICLES
Current assets	(54,633)	(62,426)
Total Assets	(54,633)	(62,426)
Current liabilities	(925)	(578)
Total Liabilities	(925)	(578)
Redeemable noncontrolling interests	35,736	28,910
Total stockholders' equity	(89,444)	(90,758)
Total Liabilities and Stockholders' Equity	$ (54,633)	$ (62,426)