Cover Page Statement - shares	9 Months Ended
	Dec. 31, 2018	Jan. 31, 2019
Document Information [Abstract]
Entity Registrant Name	LEGG MASON, INC.
Entity Central Index Key	0000704051
Document Type	10-Q
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--03-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		85,540,068
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	LM
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	false
Entity Small Business	false