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Document And Entity Information - shares
 9 Months Ended
Sep. 30, 2023
Nov. 02, 2023
Document and Entity Information [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2023  
Document Transition Report false  
Entity File Number 001-36509  
Entity Registrant Name Amphastar Pharmaceuticals, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 33-0702205  
Entity Address, Address Line One 11570 6th Street  
Entity Address, City or Town Rancho Cucamonga  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 91730  
City Area Code 909  
Local Phone Number 980-9484  
Title of 12(b) Security Common Stock, par value $0.0001 per share  
Trading Symbol AMPH  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   47,907,411
Entity Central Index Key 0001297184  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2023  
Document Fiscal Period Focus Q3  
Amendment Flag false