Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 13, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol ALT  
Entity Registrant Name Altimmune, Inc.  
Entity Central Index Key 0001326190  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Current Reporting Status Yes  
Entity Common Stock, Shares Outstanding   15,338,001
Entity File Number 001-32587  
Entity Tax Identification Number 202726770  
Entity Address, Address Line One 910 Clopper Road  
Entity Address, Address Line Two Suite 201S  
Entity Address, City or Town Gaithersburg  
Entity Address, State or Province Maryland  
Entity Address, Postal Zip Code 20878  
City Area Code (240)  
Local Phone Number 654-1450