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, 2018			285,352
Balance at beginning of period, shares at Dec. 31, 2018							(45,967)
Balance at beginning of period at Dec. 31, 2018	$ 1,855,900	$ 1,830,540	$ 285	$ 1,909,425	$ 113,189	$ 23,812	$ (216,171)	$ 25,360
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22,812	21,741			21,741			1,071
Other comprehensive income (loss)	3,431	3,431				3,431
Treasury stock acquired through surrender of shares for tax withholding, shares							(16)
Treasury stock acquired through surrender of shares for tax withholding	(130)	(130)					$ (130)
Issuance of stock through share-based compensation plans, shares			255
Issuance of stock through share-based compensation plans	871	871	$ 1	870
Share-based compensation	5,186	5,186		5,186
Subsidiary dividends to noncontrolling interests	(450)							(450)
Balance at end of period, shares at Sep. 30, 2019			285,607
Balance at end of period, shares at Sep. 30, 2019							(45,983)
Balance at end of period at Sep. 30, 2019	1,887,620	1,861,639	$ 286	1,915,481	134,930	27,243	$ (216,301)	25,981
Balance at beginning of period, shares at Jun. 30, 2019			285,510
Balance at beginning of period, shares at Jun. 30, 2019							(45,978)
Balance at beginning of period at Jun. 30, 2019	1,831,176	1,805,311	$ 285	1,913,103	80,860	27,317	$ (216,254)	25,865
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	54,486	54,070			54,070			416
Other comprehensive income (loss)	(74)	(74)				(74)
Treasury stock acquired through surrender of shares for tax withholding, shares							(5)
Treasury stock acquired through surrender of shares for tax withholding	(47)	(47)					$ (47)
Issuance of stock through share-based compensation plans, shares			97
Issuance of stock through share-based compensation plans	435	435	$ 1	434
Share-based compensation	1,944	1,944		1,944
Subsidiary dividends to noncontrolling interests	(300)							(300)
Balance at end of period, shares at Sep. 30, 2019			285,607
Balance at end of period, shares at Sep. 30, 2019							(45,983)
Balance at end of period at Sep. 30, 2019	$ 1,887,620	1,861,639	$ 286	1,915,481	134,930	27,243	$ (216,301)	25,981
Balance at beginning of period, shares at Dec. 31, 2019	286,877		286,877
Balance at beginning of period, shares at Dec. 31, 2019	(46,072)						(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 noncontrolling interests	$ (451)							(451)
Balance at end of period, shares at Sep. 30, 2020	288,043		288,043
Balance at end of period, shares at Sep. 30, 2020	(46,088)						(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 noncontrolling interests	$ (151)							(151)
Balance at end of period, shares at Sep. 30, 2020	288,043		288,043
Balance at end of period, shares at Sep. 30, 2020	(46,088)						(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