| Document and Entity Information | Oct. 22, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 22, 2020 |
| Entity Registrant Name | SCHNITZER STEEL INDUSTRIES, INC. |
| Entity Incorporation, State or Country Code | OR |
| Entity File Number | 000-22496 |
| Entity Tax Identification Number | 93-0341923 |
| Entity Address, Address Line One | 299 SW Clay Street |
| Entity Address, Address Line Two | Suite 350 |
| Entity Address, City or Town | Portland |
| Entity Address, State or Province | OR |
| Entity Address, Postal Zip Code | 97201 |
| City Area Code | 503 |
| Local Phone Number | 224-9900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000912603 |
| Title of 12(b) Security | Class A Common Stock, $1.00 par value |
| Trading Symbol | SCHN |
| Security Exchange Name | NASDAQ |
