Earnings Per Share - Computation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income attributable to Amkor common stockholders	$ 92,151	$ 54,070	$ 211,464	$ 21,741
Weighted-average number of common shares outstanding - basic	241,675	239,586	241,232	239,503
Effect of dilutive securities:
Stock options and restricted share awards (in shares)	917	351	705	355
Weighted-average number of common shares outstanding - diluted	242,592	239,937	241,937	239,858
Net income attributable to Amkor per common share:
Basic (in dollars per share)	$ 0.38	$ 0.23	$ 0.88	$ 0.09
Diluted (in dollars per share)	$ 0.38	$ 0.23	$ 0.87	$ 0.09