Earnings Per Share (Calculation of EPS) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income available to Amkor common stockholders									$ 338,138	$ 120,888	$ 127,092
Weighted-average shares outstanding — basic (in shares)									241,509	239,725	239,329
Effect of dilutive securities:
Stock options and restricted share awards (in shares)									739	397	412
Weighted-average shares outstanding — diluted (in shares)									242,248	240,122	239,741
Net income attributable to Amkor per common share:
Basic (in dollars per share)	$ 0.52	$ 0.38	$ 0.23	$ 0.27	$ 0.41	$ 0.23	$ (0.04)	$ (0.10)	$ 1.40	$ 0.50	$ 0.53
Diluted (in dollars per share)	$ 0.52	$ 0.38	$ 0.23	$ 0.26	$ 0.41	$ 0.23	$ (0.04)	$ (0.10)	$ 1.40	$ 0.50	$ 0.53