| Document and Entity Information | May 06, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 6, 2021 |
| Entity Registrant Name | Amphastar Pharmaceuticals, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36509 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Trading Symbol | AMPH |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001297184 |
| Amendment Flag | false |
