(Loss) Income per Share Attributable to Amphastar Pharmaceuticals, Inc. Shareholders (Calculation of Basic and Diluted Net Income (Loss) Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and dilutive numerator:
Net Income (Loss) Attributable to Parent	$ (192)	$ 3,949	$ 47,787	$ 868	$ 3,757	$ 48,655
Denominator:
Weighted-average shares outstanding-basic	46,753		47,107		46,581	46,925
Net effect of dilutive securities:
Incremental shares from equity awards			2,787		1,877	3,230
Weighted-average shares outstanding - diluted	46,753		49,894		48,458	50,155
Net income (loss) per share - basic	$ 0.00		$ 1.01		$ 0.08	$ 1.04
Net income (loss) per share - diluted	$ 0.00		$ 0.96		$ 0.08	$ 0.97