Variable Interest Entities and Consolidation of Investment Vehicles (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Variable Interest Entity
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	37,422	38,919
Before Consolidation
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	1,803,299	2,439,162
Non-current assets	5,030,705	5,801,680
Total assets	6,834,004	8,240,842
Current liabilities	515,207	971,804
Long-term debt of CIVs	0	0
Other non-current liabilities	1,403,381	1,603,064
Total liabilities	1,918,588	2,574,868
Redeemable noncontrolling interests	1,343	996
Total stockholders' equity	4,914,073	5,664,978
Total liabilities and equity	6,834,004	8,240,842
CIVs
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	64,112	57,714
Non-current assets	254,770	296,599
Total assets	318,882	354,313
Current liabilities	3,494	4,467
Long-term debt of CIVs	250,160	271,707
Other non-current liabilities	3,237	3,872
Total liabilities	256,891	280,046
Redeemable noncontrolling interests	0	0
Total stockholders' equity	61,991	74,267
Total liabilities and equity	318,882	354,313
Eliminations
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	(37,826)	(39,408)
Non-current assets	0	0
Total assets	(37,826)	(39,408)
Current liabilities	(404)	(489)
Long-term debt of CIVs	0	0
Other non-current liabilities	0	0
Total liabilities	(404)	(489)
Redeemable noncontrolling interests	20,333	23,035
Total stockholders' equity	(57,755)	(61,954)
Total liabilities and equity	(37,826)	(39,408)
Legg Mason, Inc.
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	1,829,585	2,457,468
Non-current assets	5,285,475	6,098,279
Total assets	7,115,060	8,555,747
Current liabilities	518,297	975,782
Long-term debt of CIVs	250,160	271,707
Other non-current liabilities	1,406,618	1,606,936
Total liabilities	2,175,075	2,854,425
Redeemable noncontrolling interests	21,676	24,031
Total stockholders' equity	4,918,309	5,677,291
Total liabilities and equity	$ 7,115,060	$ 8,555,747