Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 05, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Trading Symbol FTSV  
Entity Registrant Name Forty Seven, Inc.  
Entity Central Index Key 0001667633  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   42,276,130
Entity Shell Company false  
Entity Current Reporting Status Yes  
Entity Incorporation, State or Country Code DE  
Entity File Number 001-38554  
Entity Tax Identification Number 47-4065674  
Entity Address, Address Line One 1490 O’Brien Drive  
Entity Address, Address Line Two Suite A  
Entity Address, City or Town Menlo Park  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 94025  
City Area Code 650  
Local Phone Number 352-4150  
Document Quarterly Report true  
Document Transition Report false  
Entity Interactive Data Current Yes  
Title of 12(b) Security Common Stock, $0.0001 par value  
Security Exchange Name NASDAQ