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Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (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, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit Obligation at Beginning of Year | $ 369,808 | $ 349,483 |
Company Service Cost | 9,556 | 9,130 | $ 10,256
Interest Cost | 15,475 | 15,906 | 15,847
Plan Participants' Contributions | 0 | 0 |
Net Actuarial (Gain)/Loss due to Assumption Changes | 38,496 | 14,450 |
Net Actuarial (Gain)/Loss due to Plan Experience | 2,338 | 5,428 |
Benefit Payments from Fund | (17,578) | (21,831) |
Benefit Payments Directly by Company | (335) | (2,669) |
Plan Amendments | 10 | 16 |
Other Adjustment | 0 | (105) |
Benefit Obligation at End of Year | 417,770 | 369,808 | 349,483
Other Postretirement Benefit Plan |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit Obligation at Beginning of Year | 17,378 | 16,423 |
Company Service Cost | 813 | 751 | 833
Interest Cost | 706 | 704 | 697
Plan Participants' Contributions | 395 | 408 |
Net Actuarial (Gain)/Loss due to Assumption Changes | 5,981 | 497 |
Net Actuarial (Gain)/Loss due to Plan Experience | 924 | 357 |
Benefit Payments from Fund | (1,404) | (1,678) |
Benefit Payments Directly by Company | 0 | 0 |
Plan Amendments | 0 | 0 |
Other Adjustment | (77) | (84) |
Benefit Obligation at End of Year | $ 24,716 | $ 17,378 | $ 16,423

X
- Definition The net increase or decrease of changes in the value of either the benefit obligation or the plan assets resulting from a change in an actuarial assumption. + References No definition available. + Details
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- Definition The net increase or decrease of changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed. + References No definition available. + Details
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- Definition The amount of payments made directly by the entity for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets. + References No definition available. + Details
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- Definition The amount of payments made from fund for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets. + References No definition available. + Details
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- Definition Amount of 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 (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contributions received by defined benefit plan from participant which increase benefit obligation. + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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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. + References No definition available. + 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 increase (decrease) in benefit obligation of defined benefit plan from change, classified as other. + References No definition available. + Details
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- Definition Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new 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 (a)(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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