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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income attributable to Amkor common stockholders	$ 180,854	$ 92,151	$ 426,469	$ 211,464
Weighted-average number of common shares outstanding - basic	244,100	241,675	243,746	241,232
Effect of dilutive securities:
Share-based awards (in shares)	1,842	917	1,865	705
Weighted-average number of common shares outstanding - diluted	245,942	242,592	245,611	241,937
Net income attributable to Amkor per common share:
Basic (in dollars per share)	$ 0.74	$ 0.38	$ 1.75	$ 0.88
Diluted (in dollars per share)	$ 0.74	$ 0.38	$ 1.74	$ 0.87