Pensions and Other Post-retirement Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11,023	$ 10,417	$ 11,517
Interest cost	18,450	18,752	18,314
Expected return on plan assets	(35,417)	(34,751)	(34,130)
Amortization of transition amounts	2	2	2
Amortization of prior service (credit) cost	(19)	(14)	66
Recognized net actuarial losses	12,955	11,921	15,545
Settlement/curtailment loss (credit)	148	5	641
Special termination charge	11,384	0	0
Net periodic benefit cost	18,526	6,332	11,955
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	403	426	444
Interest cost	882	946	863
Expected return on plan assets	0	0	0
Amortization of transition amounts	0	0	0
Amortization of prior service (credit) cost	(307)	(419)	(335)
Recognized net actuarial losses	100	68	27
Settlement/curtailment loss (credit)	(562)	0	0
Special termination charge	0	0	0
Net periodic benefit cost	$ 516	$ 1,021	$ 999

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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- Definition

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

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- Definition

Amount of transition asset (obligation) recognized in net periodic benefit (cost) credit of defined benefit plan.

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- Definition

Amount of cost of providing special or contractual termination benefits payable from defined benefit plan.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of net periodic benefit cost (credit) for defined benefit plan.

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- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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