Accumulated Other Comprehensive Loss - Net Change and Ending Balances for Various Components of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 91,608	$ 61,185	$ 84,999	$ 55,616
Net change	(2,079)	4,775	(900)	4,135
Ending Balance	88,661	85,774	88,661	85,774
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(11,371)	(18,352)
Net change			(2,041)	2,750
Ending Balance	(13,412)	(15,602)	(13,412)	(15,602)
Unrecognized Employee Benefit Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(57,652)	(50,859)
Net change			1,618	1,191
Ending Balance	(56,034)	(49,668)	(56,034)	(49,668)
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			589	291
Net change			(477)	194
Ending Balance	112	485	112	485
Total Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(68,434)	(68,920)
Ending Balance	(69,334)	(64,785)	(69,334)	(64,785)
Less: Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			261	(258)
Net change			103	191
Ending Balance	364	(67)	364	(67)
Accumulated Other Comprehensive Loss Attributable to Ampco-Pittsburgh [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(68,695)	(68,662)
Net change			(1,003)	3,944
Ending Balance	$ (69,698)	$ (64,718)	$ (69,698)	$ (64,718)