Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 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 GTHX  
Entity Registrant Name G1 THERAPEUTICS, INC.  
Entity Central Index Key 0001560241  
Current Fiscal Year End Date --12-31  
Title of 12(b) Security Common Stock, par value $0.0001 per share  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   37,581,512
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity File Number 001-38096  
Entity Address, Address Line One 700 Park Offices Drive  
Entity Address, Address Line Two Suite 200  
Entity Address, City or Town Research Triangle Park  
Entity Address, State or Province NC  
Entity Address, Postal Zip Code 27709  
City Area Code (919)  
Local Phone Number 213-9835  
Entity Tax Identification Number 26-3648180  
Document Quarterly Report true  
Document Transition Report false  
Entity Interactive Data Current Yes