Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 06, 2019
Document And Entity Information [Abstract]    
Entity Registrant Name Oncternal Therapeutics, Inc.  
Entity Central Index Key 0001260990  
Document Type 10-Q  
Document Period End Date Jun. 30, 2019  
Amendment Flag false  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol ONCT  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Current Reporting Status Yes  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   15,384,764
Entity File Number 000-50549  
Entity Tax Identification Number 464137807  
Entity Address, Address Line One 12230 El Camino Real  
Entity Address, Address Line Two Suite 300  
Entity Address, City or Town San Diego  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 92130  
City Area Code 858  
Local Phone Number 434-1113