v3.19.1
Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Mar. 06, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Entity Registrant Name Alpine Immune Sciences, Inc.    
Trading Symbol ALPN    
Entity Central Index Key 0001626199    
Current Fiscal Year End Date --12-31    
Entity Filer Category Non-accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Small Business true    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding (in shares)   18,561,029  
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 33.8