Accumulated Other Comprehensive Loss - Line Items Affected on Consolidated Statements of Operations for Components Reclassified from Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net sales (foreign currency sales contracts)	$ 114,449	$ 103,886	$ 110,550	$ 103,516	$ 92,126	$ 82,861	$ 93,301	$ 63,578	$ 432,401	$ 331,866	$ 238,480
Depreciation and amortization (foreign currency purchasecontracts)									(22,387)	(20,463)	(11,787)
Costs of products sold (excluding depreciation and amortization)									(357,672)	(276,496)	(196,091)
Selling and administrative									(61,310)	(58,175)	(39,510)
Other expense									(932)	(1,074)	(475)
Income tax provision									1,355	(22,712)	(2,633)
Net (loss) income attributable to Ampco-Pittsburgh	$ (3,191)	$ (2,202)	$ (1,913)	$ (4,783)	$ (43,062)	$ (27,382)	$ (6,486)	$ (2,890)	(12,089)	(79,820)	1,373
Amortization of Unrecognized Employee Benefit Costs [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Costs of products sold (excluding depreciation and amortization)									1,934	2,463	3,604
Selling and administrative									1,148	(719)	3,354
Other expense									201	166	355
Total before income tax									3,283	1,910	7,313
Income tax provision									0	0	(2,573)
Net (loss) income attributable to Ampco-Pittsburgh									3,283	1,910	4,740
Realized Gains from Sale of Marketable Securities [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Selling and administrative									(29)	(1,404)	(82)
Income tax provision									0	366	29
Net (loss) income attributable to Ampco-Pittsburgh									(29)	(1,038)	(53)
Realized Gains/Losses from Settlement of Cash Flow Hedges [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net sales (foreign currency sales contracts)									0	(6)	(17)
Depreciation and amortization (foreign currency purchasecontracts)									(31)	(27)	(27)
Costs of products sold (excluding depreciation and amortization)									(639)	220	751
Total before income tax									(670)	187	707
Income tax provision									0	(79)	(272)
Net (loss) income attributable to Ampco-Pittsburgh									$ (670)	$ 108	$ 435