v3.10.0.1
Document And Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 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 Filer Category Accelerated Filer  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Entity Common Stock, Shares Outstanding   46,098,467