| Cover | Sep. 25, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 25, 2020 |
| Entity Registrant Name | MAGNITE, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36384 |
| Entity Tax Identification Number | 20-8881738 |
| Entity Address, Address Line One | 12181 Bluff Creek Drive |
| Entity Address, Postal Zip Code | 90094 |
| Entity Address, Address Line Two | 4th Floor |
| Entity Address, City or Town | Los Angeles |
| Entity Address, State or Province | CA |
| Local Phone Number | 207-0272 |
| City Area Code | 310 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | MGNI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001595974 |
| Amendment Flag | false |
