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Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Company Service Cost | $ 9,556 | $ 9,130 | $ 10,256
Interest Cost | 15,475 | 15,906 | 15,847
Expected Return on Assets | (20,099) | (19,508) | (21,109)
Other Adjustments | 0 | 0 | 0
Subtotal | 4,932 | 5,528 | 4,994
Amortization of Net Transition Obligation/(Asset) | 0 | 0 | 0
Amortization of Net Prior Service Cost/(Credit) | (426) | (687) | (845)
Amortization of Net Losses/(Gains) | 6,169 | 5,856 | 5,485
Total Amortization | 5,743 | 5,169 | 4,640
Net Periodic Benefit Cost | 10,675 | 10,697 | 9,634
Cost of settlements | 0 | 1,277 | 3,172
Total Expense for Year | 10,675 | 11,974 | 12,806
Other Postretirement Benefit Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Company Service Cost | 813 | 751 | 833
Interest Cost | 706 | 704 | 697
Expected Return on Assets | (5,248) | (4,886) | (4,991)
Other Adjustments | 0 | 0 | 0
Subtotal | (3,729) | (3,431) | (3,461)
Amortization of Net Transition Obligation/(Asset) | 0 | 0 | 0
Amortization of Net Prior Service Cost/(Credit) | (6,649) | (6,649) | (6,649)
Amortization of Net Losses/(Gains) | 0 | 0 | (175)
Total Amortization | (6,649) | (6,649) | (6,824)
Net Periodic Benefit Cost | (10,378) | (10,080) | (10,285)
Cost of settlements | 0 | 0 | 0
Total Expense for Year | $ (10,378) | $ (10,080) | $ (10,285)

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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period, before the effect of the cost of settlements and curtailments. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, and transition asset or obligation. + References No definition available. + Details
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- Definition The subtotal amount of net periodic benefit cost for defined benefit plans that may include the following components: service cost, interest cost, expected return on plan assets, and other adjustments. + References No definition available. + Details
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- Definition Total amortization included in the amount of net periodic benefit cost that may include the following components: 1) gains or losses recognized in net periodic benefit cost; 2) transition obligation or asset recognized in net periodic benefit cost; and 3) prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit of 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)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of 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)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of transition asset (obligation) recognized in net periodic benefit (cost) credit of 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)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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. + References No definition available. + Details
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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 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)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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 + Details
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- Definition Amount of net periodic benefit cost (credit) 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) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of defined benefit plan cost (credit), classified as other. + References No definition available. + Details
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- Definition Amount of gain (loss) recognized in net periodic benefit (cost) credit from settlement and curtailment. + 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)(7) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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 + Details
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- Details
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