v3.8.0.1
Document And Entity Information - shares
3 Months Ended
Mar. 31, 2018
May 03, 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 Mar. 31, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Entity Common Stock, Shares Outstanding   46,614,060