Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 03, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	Amphastar Pharmaceuticals, Inc.
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, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q1
Entity Common Stock, Shares Outstanding		45,737,887