v3.10.0.1
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 12, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name Y-mAbs Therapeutics, Inc.  
Entity Central Index Key 0001722964  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status No  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   34,193,666
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3