Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Total Amkor Stockholders’ Equity	Common Stock	Additional Paid- In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Noncontrolling Interest in Subsidiaries
Balance at beginning of period, shares at Dec. 31, 2019			286,877
Balance at beginning of period, shares at Dec. 31, 2019							(46,072)
Balance at beginning of period at Dec. 31, 2019	$ 1,990,239	$ 1,963,739	$ 287	$ 1,927,739	$ 234,077	$ 19,115	$ (217,479)	$ 26,500
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	213,534	211,464			211,464			2,070
Other comprehensive income (loss)	4,194	4,194				4,194
Treasury stock acquired through surrender of shares for tax withholding, shares							(16)
Treasury stock acquired through surrender of shares for tax withholding	(181)	(181)					$ (181)
Issuance of stock through share-based compensation plans, shares			1,166
Issuance of stock through share-based compensation plans	9,695	9,695	$ 1	9,694
Share-based compensation	5,707	5,707		5,707
Subsidiary dividends to non-controlling interests	(451)							(451)
Balance at end of period, shares at Sep. 30, 2020			288,043
Balance at end of period, shares at Sep. 30, 2020							(46,088)
Balance at end of period at Sep. 30, 2020	2,222,737	2,194,618	$ 288	1,943,140	445,541	23,309	$ (217,660)	28,119
Balance at beginning of period, shares at Jun. 30, 2020			287,319
Balance at beginning of period, shares at Jun. 30, 2020							(46,083)
Balance at beginning of period at Jun. 30, 2020	2,116,928	2,089,404	$ 287	1,934,047	353,390	19,272	$ (217,592)	27,524
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	92,897	92,151			92,151			746
Other comprehensive income (loss)	4,037	4,037				4,037
Treasury stock acquired through surrender of shares for tax withholding, shares							(5)
Treasury stock acquired through surrender of shares for tax withholding	(68)	(68)					$ (68)
Issuance of stock through share-based compensation plans, shares			724
Issuance of stock through share-based compensation plans	6,954	6,954	$ 1	6,953
Share-based compensation	2,140	2,140		2,140
Subsidiary dividends to non-controlling interests	(151)							(151)
Balance at end of period, shares at Sep. 30, 2020			288,043
Balance at end of period, shares at Sep. 30, 2020							(46,088)
Balance at end of period at Sep. 30, 2020	$ 2,222,737	2,194,618	$ 288	1,943,140	445,541	23,309	$ (217,660)	28,119
Balance at beginning of period, shares at Dec. 31, 2020	288,923		288,923
Balance at beginning of period, shares at Dec. 31, 2020	(46,094)						(46,094)
Balance at beginning of period at Dec. 31, 2020	$ 2,353,959	2,325,699	$ 289	1,953,378	562,502	27,270	$ (217,740)	28,260
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	428,324	426,469			426,469			1,855
Other comprehensive income (loss)	(11,334)	(11,334)				(11,334)
Treasury stock acquired through surrender of shares for tax withholding, shares							(57)
Treasury stock acquired through surrender of shares for tax withholding	(1,321)	(1,321)					$ (1,321)
Issuance of stock through share-based compensation plans, shares			1,418
Issuance of stock through share-based compensation plans	11,852	11,852	$ 1	11,851
Share-based compensation	7,978	7,978		7,978
Cash dividends declared	(29,330)	(29,330)			(29,330)
Subsidiary dividends to non-controlling interests	$ (449)							(449)
Balance at end of period, shares at Sep. 30, 2021	290,341		290,341
Balance at end of period, shares at Sep. 30, 2021	(46,151)						(46,151)
Balance at end of period at Sep. 30, 2021	$ 2,759,679	2,730,013	$ 290	1,973,207	959,641	15,936	$ (219,061)	29,666
Balance at beginning of period, shares at Jun. 30, 2021			290,115
Balance at beginning of period, shares at Jun. 30, 2021							(46,151)
Balance at beginning of period at Jun. 30, 2021	2,583,070	2,554,248	$ 290	1,968,339	788,567	16,113	$ (219,061)	28,822
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	181,847	180,854			180,854			993
Other comprehensive income (loss)	(177)	(177)				(177)
Issuance of stock through share-based compensation plans, shares			226
Issuance of stock through share-based compensation plans	1,828	1,828		1,828
Share-based compensation	3,040	3,040		3,040
Cash dividends declared	(9,780)	(9,780)			(9,780)
Subsidiary dividends to non-controlling interests	$ (149)							(149)
Balance at end of period, shares at Sep. 30, 2021	290,341		290,341
Balance at end of period, shares at Sep. 30, 2021	(46,151)						(46,151)
Balance at end of period at Sep. 30, 2021	$ 2,759,679	$ 2,730,013	$ 290	$ 1,973,207	$ 959,641	$ 15,936	$ (219,061)	$ 29,666