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Benefit Plans - Additional Disclosures (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Other Postretirement Benefit Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Company Contributions | $ 0 | $ 0
Current plus 1 | 0 |
Actual Benefit Payments for the Year Ending: Current | 1,086 |
Expected Benefit Payments for the Year Ending: Current plus 1 | 1,174 |
Expected Benefit Payments for the Year Ending: Current plus 2 | 1,368 |
Expected Benefit Payments for the Year Ending: Current plus 3 | 1,689 |
Expected Benefit Payments for the Year Ending: Current plus 4 | 1,944 |
Expected Benefit Payments for the Year Ending: Current plus 5 | 2,044 |
Expected Benefit Payments for the Year Ending: Current plus 6 - 10 | 11,914 |
Effect of 1-percentage point increase on total service and interest cost component | 252 |
Effect of 1-percentage point decrease on total service and interest cost components | (217) |
Effect of 1-percentage point increase on postretirement benefit obligation | 3,093 |
Effect of 1-percentage point decrease on postretirement benefit obligation | $ (2,748) |
Other Postretirement Benefit Plan | Equity securities | Minimum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 70.00% |
Other Postretirement Benefit Plan | Equity securities | Maximum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 100.00% |
Other Postretirement Benefit Plan | Real Estate | Minimum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 0.00% |
Other Postretirement Benefit Plan | Real Estate | Maximum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 15.00% |
Other Postretirement Benefit Plan | Commodities | Minimum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 0.00% |
Other Postretirement Benefit Plan | Commodities | Maximum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 10.00% |
Other Postretirement Benefit Plan | Fixed Income/Cash | Minimum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 0.00% |
Other Postretirement Benefit Plan | Fixed Income/Cash | Maximum |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target asset allocations (in hundredths) | 10.00% |
Pension and Supplemental Executive Retirement Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Company Contributions | $ 9,435 | $ 11,369
Current plus 1 | 10,950 |
Actual Benefit Payments for the Year Ending: Current | 17,913 |
Expected Benefit Payments for the Year Ending: Current plus 1 | 28,639 |
Expected Benefit Payments for the Year Ending: Current plus 2 | 33,127 |
Expected Benefit Payments for the Year Ending: Current plus 3 | 29,669 |
Expected Benefit Payments for the Year Ending: Current plus 4 | 31,085 |
Expected Benefit Payments for the Year Ending: Current plus 5 | 31,899 |
Expected Benefit Payments for the Year Ending: Current plus 6 - 10 | $ 151,512 |

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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 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)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + 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 (d)(5)(ii) -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 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + 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 (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate. + 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 (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate. + 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 (m) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest 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 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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