| Document and Entity Information | Mar. 15, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001595974 |
| Document Type | 8-K |
| Document Period End Date | Mar. 15, 2021 |
| Entity Registrant Name | MAGNITE, INC. |
| Entity Incorporation State 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, Address Line Two | 4th Floor |
| Entity Address, City or Town | Los Angeles |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 90094 |
| City Area Code | (310) |
| Local Phone Number | 207-0272 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.00001 per share |
| Trading Symbol | MGNI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
