Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,600	$ 2,300	$ 5,100	$ 4,600
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,405	2,145	4,818	4,296
Interest cost	12,307	13,495	24,650	27,008
Expected return on plan assets	(21,553)	(20,803)	(43,178)	(41,640)
Amortization of prior service cost (credit)	550	525	1,100	1,050
Net periodic benefit (income) cost	(6,291)	(4,638)	(12,610)	(9,286)
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	162	153	327	307
Interest cost	351	538	900	1,078
Expected return on plan assets	(399)	(596)	(1,033)	(1,191)
Amortization of prior service cost (credit)	(62)	(78)	(107)	(157)
Net periodic benefit (income) cost	$ 52	$ 17	$ 87	$ 37

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

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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.

+ References

No definition available.

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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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- Definition

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.

+ References

+ Details

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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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