Document and Entity Information - shares
3 Months Ended
Mar. 31, 2018
May 04, 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  
Document Type 10-Q  
Document Period End Date Mar. 31, 2018  
Entity Filer Category Smaller Reporting Company  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Common Stock, Shares Outstanding (in shares)   13,849,562