Document And Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Mar. 08, 2019
Jun. 30, 2018
Document and Entity Information [Abstract]      
Entity Registrant Name Amphastar Pharmaceuticals, Inc.    
Trading Symbol amph    
Document Type 10-K    
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 Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Entity Common Stock, Shares Outstanding   46,788,811  
Entity Public Float     $ 454,913,022
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Shell Company false