| Document and Entity Information | Jul. 31, 2020 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0000704051 |
| Current Fiscal Year End Date | --03-31 |
| Document Type | 8-K |
| Document Period End Date | Jul. 31, 2020 |
| Entity Registrant Name | Legg Mason, Inc. |
| Entity Incorporation State Country Code | MD |
| Entity File Number | 1-8529 |
| Entity Tax Identification Number | 52-1200960 |
| Entity Address, Address Line One | 100 International Drive |
| Entity Address, City or Town | Baltimore |
| Entity Address, State or Province | MD |
| Entity Address, Postal Zip Code | 21202 |
| City Area Code | (410) |
| Local Phone Number | 539-0000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common stock, $0.10 par value |
| Trading Symbol | LM |
| Security Exchange Name | NYSE |
| M 6.375 Junior Subordinated Notes Due 2056 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 6.375% Junior Subordinated Notes due 2056 |
| Trading Symbol | LMHA |
| Security Exchange Name | NYSE |
| M 5.45 Junior Subordinated Notes Due 2056 [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.45% Junior Subordinated Notes due 2056 |
| Trading Symbol | LMHB |
| Security Exchange Name | NYSE |
