v3.19.3
Cover Page - shares
9 Months Ended
Sep. 30, 2019
Nov. 04, 2019
Cover page.    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2019  
Document Transition Report false  
Entity File Number 001-38295  
Entity Registrant Name X4 PHARMACEUTICALS, INC  
Entity Central Index Key 0001501697  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 27-3181608  
Entity Address, Address Line One 955 Massachusetts Avenue  
Entity Address, Address Line Two 4th Floor  
Entity Address, City or Town Cambridge  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02139  
City Area Code 857  
Local Phone Number 529-8300  
Title of 12(b) Security Common Stock  
Trading Symbol XFOR  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   12,456,418