Cover Page Statement - USD ($)	12 Months Ended
	Mar. 31, 2019	May 22, 2019	Sep. 30, 2018
Document Information [Abstract]
Entity Registrant Name	LEGG MASON, INC.
Entity Central Index Key	0000704051
Document Type	10-K
Document Period End Date	Mar. 31, 2019
Amendment Flag	false
Current Fiscal Year End Date	--03-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		86,599,587
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q4
Trading Symbol	LM
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 2,617,265,877
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false