Variable Interest Entities and Consolidation of Investment Vehicles (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2012 Before Consolidation	Mar. 31, 2012 Before Consolidation	Jun. 30, 2012 CIVs	Mar. 31, 2012 CIVs	Jun. 30, 2012 Eliminations	Mar. 31, 2012 Eliminations	Jun. 30, 2012 Legg Mason, Inc.	Mar. 31, 2012 Legg Mason, Inc.	Jun. 30, 2011 Legg Mason, Inc.	Mar. 31, 2011 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	37,164	38,919
Current assets				1,658,681	2,439,162	57,902	57,714	(37,577)	(39,408)	1,679,006	2,457,468
Non-current assets				5,821,363	5,801,680	298,638	296,599	0	0	6,120,001	6,098,279
Total assets				7,480,044	8,240,842	356,540	354,313	(37,577)	(39,408)	7,799,007	8,555,747
Current liabilities				495,045	971,804	12,582	4,467	(413)	(489)	507,214	975,782
Long-term debt of CIVs				0	0	273,560	271,707	0	0	273,560	271,707
Other non-current liabilities				1,536,721	1,603,064	3,620	3,872	0	0	1,540,341	1,606,936
Total liabilities				2,031,766	2,574,868	289,762	280,046	(413)	(489)	2,321,115	2,854,425
Redeemable noncontrolling interests				1,335	996	0	0	20,167	23,035	21,502	24,031	47,991	36,712
Total stockholders' equity				5,446,943	5,664,978	66,778	74,267	(57,331)	(61,954)	5,456,390	5,677,291	5,746,398
Total liabilities and equity				$ 7,480,044	$ 8,240,842	$ 356,540	$ 354,313	$ (37,577)	$ (39,408)	$ 7,799,007	$ 8,555,747