| Cover | Jul. 20, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jul. 20, 2021 |
| Entity File Number | 001-13122 |
| Entity Registrant Name | RELIANCE STEEL & ALUMINUM CO. |
| Entity Central Index Key | 0000861884 |
| Entity Tax Identification Number | 95-1142616 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 350 South Grand Avenue |
| Entity Address, Address Line Two | Suite 5100 |
| Entity Address, City or Town | Los Angeles |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 90071 |
| City Area Code | 213 |
| Local Phone Number | 687-7700 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.001 par value |
| Trading Symbol | RS |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
