Variable Interest Entities and Consolidation of Investment Vehicles (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
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	36,821	38,919
Before Consolidation
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	1,810,259	2,439,162
Non-current assets	5,753,759	5,801,680
Total assets	7,564,018	8,240,842
Current liabilities	598,673	971,804
Long-term debt of CIVs	0	0
Other non-current liabilities	1,516,713	1,603,064
Total liabilities	2,115,386	2,574,868
Redeemable noncontrolling interests	1,398	996
Total stockholders' equity	5,447,234	5,664,978
Total liabilities and equity	7,564,018	8,240,842
CIVs
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	58,216	57,714
Non-current assets	289,880	296,599
Total assets	348,096	354,313
Current liabilities	1,035	4,467
Long-term debt of CIVs	277,309	271,707
Other non-current liabilities	3,514	3,872
Total liabilities	281,858	280,046
Redeemable noncontrolling interests	0	0
Total stockholders' equity	66,238	74,267
Total liabilities and equity	348,096	354,313
Eliminations
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	(37,215)	(39,408)
Non-current assets	0	0
Total assets	(37,215)	(39,408)
Current liabilities	(394)	(489)
Long-term debt of CIVs	0	0
Other non-current liabilities	0	0
Total liabilities	(394)	(489)
Redeemable noncontrolling interests	20,151	23,035
Total stockholders' equity	(56,972)	(61,954)
Total liabilities and equity	(37,215)	(39,408)
Legg Mason, Inc.
Variable Interest Entities and Consolidation of Investment Vehicles
Current assets	1,831,260	2,457,468
Non-current assets	6,043,639	6,098,279
Total assets	7,874,899	8,555,747
Current liabilities	599,314	975,782
Long-term debt of CIVs	277,309	271,707
Other non-current liabilities	1,520,227	1,606,936
Total liabilities	2,396,850	2,854,425
Redeemable noncontrolling interests	21,549	24,031
Total stockholders' equity	5,456,500	5,677,291
Total liabilities and equity	$ 7,874,899	$ 8,555,747