New Accounting Standards - Revenue from Contracts with Customers (Topic 606) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 1,144,192	$ 1,148,884	$ 3,235,195	$ 3,056,553
Cost of sales	943,485	924,996	2,707,000	2,519,815
Gross profit	200,707	223,888	528,195	536,738
Income tax expense	14,326	21,263	27,438	53,404
Net income	57,292	60,458	100,671	166,520
Net income attributable to Amkor	$ 56,662	$ 59,264	$ 98,797	$ 163,491
Net loss attributable to Amkor per common share - diluted (in dollars per share)	$ 0.24	$ 0.25	$ 0.41	$ 0.68
Accounts receivable, net	$ 797,678		$ 797,678		$ 798,264
Inventories	228,108		228,108		213,649
Other assets	141,440		141,440		139,796
Accrued expenses	271,067		271,067		330,868
Other non-current liabilities	54,403		54,403		47,823
Accumulated deficit	$ 84,894		$ 84,894		(13,903)
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 1,135,027		$ 3,038,074
Cost of sales		918,389		2,506,295
Gross profit		216,638		531,779
Income tax expense		18,752		51,764
Net income		55,630		162,945
Net income attributable to Amkor		$ 54,435		$ 159,936
Net loss attributable to Amkor per common share - diluted (in dollars per share)		$ 0.23		$ 0.67
Accounts receivable, net					692,287
Inventories					326,492
Other assets					146,051
Accrued expenses					374,598
Other non-current liabilities					46,144
Accumulated deficit					(42,851)
New Accounting Pronouncement Adjustment | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 13,857		$ 18,479
Cost of sales		6,607		13,520
Gross profit		7,250		4,959
Income tax expense		2,511		1,640
Net income		4,828		3,575
Net income attributable to Amkor		$ 4,829		$ 3,555
Net loss attributable to Amkor per common share - diluted (in dollars per share)		$ 0.02		$ 0.01
Accounts receivable, net					105,977
Inventories					(112,843)
Other assets					(6,255)
Accrued expenses					(43,730)
Other non-current liabilities					1,679
Accumulated deficit					28,948
New Accounting Pronouncement Adjustment | ASU 2014-09 | Adjustment to Accumulated Deficit
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Adjustment to accumulated deficit					$ 2,800	$ 11,300	$ 14,800