Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2015
Mar. 21, 2016
Jun. 30, 2015
Document and Entity Information [Abstract]      
Entity Registrant Name Global Blood Therapeutics, Inc.    
Entity Central Index Key 0001629137    
Document Type 10-K    
Document Fiscal Period Focus FY    
Document Period End Date Dec. 31, 2015    
Document Fiscal Year Focus 2015    
Current Fiscal Year End Date --12-31    
Entity Filer Category Non-accelerated Filer    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   30,527,075  
Entity Public Float     $ 0
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes