| Cover | Jan. 15, 2025 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001699031 |
| Document Type | 8-K |
| Document Period End Date | Jan. 15, 2025 |
| Entity Registrant Name | GRAIL, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-42045 |
| Entity Tax Identification Number | 86-3673636 |
| Entity Address, Address Line One | 1525 O’Brien Drive |
| Entity Address, City or Town | Menlo Park |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94025 |
| City Area Code | 833 |
| Local Phone Number | 694-2553 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | GRAL |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
