Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Pensions [Member]
Pension and other postretirement benefits expenses
Service cost	$ 1,590	$ 1,643
Interest cost	4,309	4,621
Expected return on plan assets	(7,394)	(6,860)
Amortization of prior service cost (credit)	141	109
Amortization of actuarial losses	2,811	2,480
Settlement gain	0	(261)
Net periodic benefit cost	1,457	1,732
Expected company contributions in 2018	4,600
Pensions [Member] \| Qualified Plan [Member]
Pension and other postretirement benefits expenses
Expected company contributions in 2018	0
Other Postretirement Benefits [Member]
Pension and other postretirement benefits expenses
Service cost	24	22
Interest cost	344	389
Amortization of prior service cost (credit)	5	(17)
Amortization of actuarial losses	160	69
Net periodic benefit cost	$ 533	$ 463

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

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

+ References

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- Definition

Amount of contributions expected to be received by defined benefit plan from employer in fiscal year. Excludes contributions paid by employer in current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

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- Definition

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.

+ References

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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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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

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-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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