Earnings Per Share - Computation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income attributable to Amkor common stockholders	$ 119,809	$ 63,889
Weighted-average number of common shares outstanding - basic	243,267	240,919
Effect of dilutive securities:
Stock options and restricted share awards (in shares)	1,862	414
Weighted-average number of common shares outstanding - diluted	245,129	241,333
Net income attributable to Amkor per common share:
Basic (in dollars per share)	$ 0.49	$ 0.27
Diluted (in dollars per share)	$ 0.49	$ 0.26