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Benefit Plans - Actuarial Assumptions and Year-End Asset Allocations (Details) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Pension and Supplemental Executive Retirement Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount Rate (in hundredths) | 3.75% | 4.30%
Rate of Compensation Increase (in hundredths) | 3.00% | 3.00%
Discount Rate (in hundredths) | 4.30% | 4.65%
Expected Long-term Return on Plan Assets (in hundredths) | 5.75% | 5.75%
Rate of Compensation Increase (in hundredths) | 3.00% | 3.00%
Other Postretirement Benefit Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Discount Rate (in hundredths) | 3.55% | 3.95%
Discount Rate (in hundredths) | 3.95% | 4.30%
Expected Long-term Return on Plan Assets (in hundredths) | 7.50% | 7.50%
Health Care Cost Trend Rate Assumed for Next Year (in hundredths) | 6.50% | 6.50%
Rate to Which the Cost Trend Rate is Assumed to Decline (Ultimate Trend Rate) (in hundredths) | 5.00% | 5.00%
Year That the Rate Reaches the Ultimate Trend Rate | 2024 | 2020
Weighted-average asset allocations of plans (in hundredths) | 100.00% | 100.00%
Other Postretirement Benefit Plan | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted-average asset allocations of plans (in hundredths) | 100.00% | 100.00%
Other Postretirement Benefit Plan | Total debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted-average asset allocations of plans (in hundredths) | 0.00% | 0.00%
Pension Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted-average asset allocations of plans (in hundredths) | 100.00% | 100.00%
Pension Plan | Equity securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted-average asset allocations of plans (in hundredths) | 21.00% | 23.00%
Pension Plan | Total debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Weighted-average asset allocations of plans (in hundredths) | 79.00% | 77.00%

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related 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 (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost 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 (k)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related 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 (k)(2) -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 Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + 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 (l) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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