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Income (Loss) per Share Attributable to Amphastar Pharmaceuticals, Inc. Stockholders (Calculation of Basic and Diluted Net Income (Loss) Per Share) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2021
Sep. 30, 2020
Basic and dilutive numerator:                
Net Income (Loss) Attributable to Parent $ 29,548 $ 7,767 $ 5,041 $ 3,919 $ (192) $ 3,949 $ 42,356 $ 7,676
Denominator:                
Weighted-average shares outstanding-basic 48,022     47,496     47,758 46,886
Net effect of dilutive securities:                
Incremental shares from equity awards 1,987     2,352     1,935 2,036
Weighted-average shares outstanding - diluted 50,009     49,848     49,693 48,922
Net income (loss) per share - basic $ 0.62     $ 0.08     $ 0.89 $ 0.16
Net income (loss) per share - diluted $ 0.59     $ 0.08     $ 0.85 $ 0.16