v3.19.1
Document And Entity Information - shares
3 Months Ended
Mar. 31, 2019
May 03, 2019
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 Mar. 31, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1  
Entity Common Stock, Shares Outstanding   47,065,369
Entity Small Business false  
Entity Emerging Growth Company true  
Entity Ex Transition Period true