Document and Entity Information	3 Months Ended
	Dec. 31, 2012	Jan. 31, 2013
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2012
Entity Registrant Name	LEGG MASON, INC.
Entity Central Index Key	0000704051
Current Fiscal Year End Date	--03-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		128,955,808
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
Trading Symbol	LM