Pension and Other Postretirement Benefits - Additional Information (Detail) £ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Pension_Plan Location Employee Customer	Dec. 31, 2016 USD ($) Pension_Plan	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£) Pension_Plan	Dec. 31, 2016 GBP (£) Pension_Plan
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Lump sum distributions		$ 2,739,000
Defined benefit pension contribution plan, non-elective base contribution rate	3.00%
Number of employees participate in the plan | Employee	100,000
Multiemployer plan number of employees description	Approximately 100,000
Total amount of assets collected	$ 11,700,000,000
Percentage of funded status in excess	96.00%
Participation of corporation's employees in the plan	Less than 100
Participation of number of corporation's employees in the plan | Employee	100
Corporation's contributions	$ 250,000	$ 250,000	$ 250,000
Percentage of employers contributions	5.00%
Contributions expected in 2018	$ 250,000
Service period to avail existing pension plan	15 years
Maximum maturity period of fixed income investments	10 years
Investments in diversified portfolio	Invests primarily in a diversified portfolio of fixed-income securities of varying maturities or in commingled funds which invest in a diversified portfolio of fixed-income securities of varying maturities.
Target allocation	Generate a minimum annual inflation adjusted return of 5% and outperform a traditional 70/30 equities/bond portfolio.
High-quality fixed-income investments maturity	10 years
Assumed health care cost trend rate for 2017		6.00%			6.00%
Maximum [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Defined benefit pension contribution plan, matching contribution rate	4.00%
Defined benefit plan actuarial gain loss percentage	10.00%			10.00%
Minimum [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Defined benefit plan actuarial gain loss percentage	10.00%			10.00%
Minimum [Member] | Multi-Employer Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Number of employer locations contributed to the plan | Location	1,650
Akers AB [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Employer non-elective contributions, percentage of employee compensation	3.00%
Reduction in pension liability due to exit from defined benefit plan		$ 1,181,000
Curtailment gain (loss)		887,000
Remeasurement of Pension Plan Liability [Member] | Electric Steel Corporation Carnegie [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Curtailment gain (loss)			(1,303,000)
Nonqualified Defined Benefit Pension Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Minimum contributions expected in 2018	$ 0
Estimated benefit payments, in 2018	440,000
Estimated benefit payments, in 2019	454,000
Estimated benefit payments, in 2020	464,000
Estimated benefit payments, in 2021	485,000
Estimated benefit payments, in 2022	507,000
Estimated benefit payments, in 2023-2027	2,570,000
Accumulated benefit obligations	7,202,000	6,639,000
Defined benefit plan contribution made to trusts	0		0
Fair market value included in other noncurrent assets	4,204,000	3,863,000
Other Postretirement Benefit Plans [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Curtailment gain (loss)	0	0	0
Lump sum distributions	0	0
Estimated benefit payments, in 2018	1,383,000
Estimated benefit payments, in 2019	1,407,000
Estimated benefit payments, in 2020	1,429,000
Estimated benefit payments, in 2021	1,239,000
Estimated benefit payments, in 2022	1,201,000
Estimated benefit payments, in 2023-2027	5,363,000
Fair value of plan assets	0	0	0
Voluntary contributions made	1,138,000	1,391,000
Projected benefit obligations	$ 0	17,467,000
Number of subsidiary having postretirement health care benefits | Customer	1
Monthly reimbursement period	5 years
Reduction in plan liability	$ 165,000	$ (4,762,000)
Percentage decrease in other postretirement benefits in 2018		4.75%			4.75%
Year related to decrease in health care cost trend rate		2020
Percentage point increase or decrease in the assumed health care cost trend rate		1.00%
Other Postretirement Benefit Plans [Member] | Akers AB [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Number of defined benefit pension plans | Pension_Plan		2			2
Other Postretirement Benefit Plans [Member] | Akers AB [Member] | Maximum [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Amortization period of pension liability		12 years
Other Postretirement Benefit Plans [Member] | Akers AB [Member] | Minimum [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Amortization period of pension liability		7 years 6 months
U.S. [Member] | Defined Benefit Pension Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Number of defined benefit pension plans | Pension_Plan	1			1
Minimum contributions required	$ 0
Minimum contributions expected in 2018	1,300,000
Estimated benefit payments, in 2018	13,672,000
Estimated benefit payments, in 2019	14,091,000
Estimated benefit payments, in 2020	14,301,000
Estimated benefit payments, in 2021	14,566,000
Estimated benefit payments, in 2022	14,685,000
Estimated benefit payments, in 2023-2027	74,009,000
Fair value of plan assets	199,138,000	$ 188,722,000
Accumulated benefit obligations	245,317,000	235,299,000
U.S. [Member] | Defined Benefit Pension Plan [Member] | Ampco Pittsburgh [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Minimum contributions expected in 2018	2,808,000
Voluntary contributions made	$ 2,588,000	2,466,000	882,000
U.S. [Member] | Defined Benefit Pension Plan [Member] | Akers AB [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Number of defined benefit pension plans | Pension_Plan	2			2
Foreign Pension Benefits [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Estimated benefit payments, in 2018	$ 1,644,000
Estimated benefit payments, in 2019	1,903,000
Estimated benefit payments, in 2020	1,768,000
Estimated benefit payments, in 2021	1,738,000
Estimated benefit payments, in 2022	1,670,000
Estimated benefit payments, in 2023-2027	11,196,000
Fair value of plan assets	56,419,000	48,055,000		£ 41,820	£ 38,955
Accumulated benefit obligations	57,540,000	61,277,000		£ 42,650	£ 49,673
Voluntary contributions made	1,521,000	1,522,000	1,715,000
Defined contribution plan expected to be fully funded	850,000
Contributions to defined contribution pension plan	311,000	252,000	382,000
Contributions to the defined contribution pension plan expected in 2018	$ 379,000
Foreign Pension Benefits [Member] | Akers AB [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Number of defined benefit pension plans | Pension_Plan	2			2
Estimated benefit payments, in 2018	$ 234,000
Estimated benefit payments, in 2019	257,000
Estimated benefit payments, in 2020	248,000
Estimated benefit payments, in 2021	249,000
Estimated benefit payments, in 2022	240,000
Estimated benefit payments, in 2023-2027	1,379,000
Accumulated benefit obligations	7,073,000	5,633,000
Projected benefit obligations	7,073,000	5,633,000
Foreign Pension Benefits [Member] | Defined Benefit Pension Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans [Line Items]
Curtailment gain (loss)	0	0	0
Lump sum distributions	0	0
Fair value of plan assets	56,419,000	48,055,000	$ 49,628,000
Voluntary contributions made	1,979,000	1,687,000
Projected benefit obligations	0	5,393,000
Reduction in plan liability	$ 0	$ 0