Document and Entity Information	6 Months Ended
	Sep. 30, 2011	Nov. 03, 2011
Document Information [Line Items]
Entity Registrant Name	LEGG MASON, INC.
Entity Central Index Key	0000704051
Document Type	10-Q
Document Period End Date	Sep. 30, 2011
Amendment Flag	false
Current Fiscal Year End Date	--03-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		139,772,775
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q2