v3.10.0.1
Document And Entity Information - shares
6 Months Ended
Jun. 30, 2018
Aug. 02, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name Amphastar Pharmaceuticals, Inc.  
Trading Symbol amph  
Document Type 10-Q  
Current Fiscal Year End Date --12-31  
Amendment Flag false  
Entity Central Index Key 0001297184  
Entity Current Reporting Status Yes  
Entity Voluntary Filers No  
Entity Filer Category Accelerated Filer  
Entity Well-known Seasoned Issuer No  
Document Period End Date Jun. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q2  
Entity Common Stock, Shares Outstanding   46,256,454