Document and Entity Information - shares
9 Months Ended
Sep. 30, 2017
Nov. 08, 2017
Document and Entity Information [Abstract]    
Entity Registrant Name AveXis, Inc.  
Entity Central Index Key 0001652923  
Document Type 10-Q  
Document Period End Date Sep. 30, 2017  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   31,976,418
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus Q3