Pension and Other Postretirement Benefits - Schedule of Changes in Plan Assets (Detail) £ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Foreign Pension Benefits [Member]
Change in plan assets:
Fair value of plan assets, Beginning balance	$ 48,055	£ 38,955
Corporate contributions	1,521		$ 1,522	$ 1,715
Fair value of plan assets, Ending balance	56,419	£ 41,820	48,055
Defined Benefit Pension Plan [Member] \| U.S. Defined Benefit Pension Plans [Member]
Change in plan assets:
Fair value of plan assets, Beginning balance	188,722
Fair value of plan assets, Ending balance	199,138		188,722
Defined Benefit Pension Plan [Member] \| U.S. Defined Benefit Pension Plans [Member] \| U.S. [Member]
Change in plan assets:
Fair value of plan assets, Beginning balance	188,722		139,376
Åkers acquisition – fair value of plan assets at March 3	0		50,108
Actual return on plan assets	23,366		14,656
Foreign currency exchange rate changes	0		0
Corporate contributions	403		376
Participant contributions	0		0
Plan settlements	0		(2,739)
Gross benefits paid	(13,353)		(13,055)
Fair value of plan assets, Ending balance	199,138		188,722	139,376
Defined Benefit Pension Plan [Member] \| Foreign Pension Benefits [Member]
Change in plan assets:
Fair value of plan assets, Beginning balance	48,055		49,628
Åkers acquisition – fair value of plan assets at March 3	0		0
Actual return on plan assets	3,998		7,859
Foreign currency exchange rate changes	4,673		(8,930)
Corporate contributions	1,979		1,687
Participant contributions	0		0
Plan settlements	0		0
Gross benefits paid	(2,286)		(2,189)
Fair value of plan assets, Ending balance	56,419		48,055	49,628
Other Postretirement Benefit Plans [Member]
Change in plan assets:
Fair value of plan assets, Beginning balance	0		0
Åkers acquisition – fair value of plan assets at March 3	0		0
Actual return on plan assets	0		0
Foreign currency exchange rate changes	0		0
Corporate contributions	1,138		1,391
Participant contributions	92		80
Plan settlements	0		0
Gross benefits paid	(1,230)		(1,471)
Fair value of plan assets, Ending balance	$ 0		$ 0	$ 0

Benefits paid from plan assets and from sources other than plan assets.

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- Definition

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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- Definition

Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

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Amount of increase in plan assets of defined benefit plan from business combination.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan.

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Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract.

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