v3.26.1
Document And Entity Information - shares
 3 Months Ended
Mar. 31, 2026
May 01, 2026
Document and Entity Information [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2026  
Document Transition Report false  
Securities Act 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   44,092,919
Entity Central Index Key 0001297184  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q1  
Amendment Flag false