v3.26.3
Cover - shares
 9 Months Ended
Sep. 30, 2026
Nov. 02, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2026  
Document Transition Report false  
Entity File Number 001-42045  
Entity Registrant Name GRAIL, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 86-3673636  
Entity Address, Address Line One 250 S. Mathilda Avenue,  
Entity Address, Address Line Two Suite 100  
Entity Address, City or Town Sunnyvale  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94086  
City Area Code 833  
Local Phone Number 694-2553  
Title of 12(b) Security Common stock, par value $0.001 per share  
Trading Symbol GRAL  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   44,962,171
Entity Central Index Key 0001699031  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q3  
Current Fiscal Year End --12-31  
Amendment Flag false