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Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Weighted-average assumptions used to determine benefit obligations |  |  |
Rate of compensation increase (as a percent) | 4.00% | 4.00% |
Initial pre-65 healthcare cost trend rate (as a percent) | 7.00% | 7.00% |
Initial post-65 healthcare cost trend rate (as a percent) | 4.75% | 5.00% |
Ultimate health care cost trend rate (as a percent) | 4.75% | 5.00% |
Number of years to ultimate trend rate (pre-65 participants) | 8 years | 4 years |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Initial pre-65 health care cost trend rate (as a percent) | 7.00% | 7.00% | 7.00%
Initial post-65 health care cost trend rate (as a percent) | 5.00% | 5.00% | 5.00%
Ultimate healthcare cost trend rate (as a percent) | 5.00% | 5.00% | 5.00%
Number of years to ultimate trend rate (pre-65 participants) | 4 years | 4 years | 4 years
Pension Benefits |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |
Discount rate (as a percent) | 3.65% | 4.08% |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Discount rate (as a percent) | 4.08% | 4.37% | 4.02%
Rate of compensation increase (as a percent) | 4.00% | 4.00% | 4.00%
Expected long-term return on plan assets (as a percent) | 6.55% | 6.90% | 6.90%
Other Benefits |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |
Discount rate (as a percent) | 3.71% | 4.17% |
Weighted-average assumptions used to determine net periodic benefit costs |  |  |
Discount rate (as a percent) | 4.17% | 4.52% | 4.14%
Expected long-term return on plan assets (as a percent) | 6.05% | 4.45% | 4.45%
Effects of one percentage point change in the assumed initial and ultimate health care cost trend rates |  |  |
Effect of 1% increase on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants | $ 8,424 |  |
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants | (5,616) |  |
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs | 9,145 |  |
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs | (7,037) |  |
Effect of 1% increase on the accumulated other postretirement benefit obligation | 128,203 |  |
Effect of 1% decrease on the accumulated other postretirement benefit obligation | $ (98,143) |  |

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- Definition Defined Benefit Plan, Post-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year, Used in Calculating Benefit Costs + References No definition available. + Details
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- Definition Defined Benefit Plan, Post-65 Health Care Cost Trend Rate Assumed for Next Fiscal Year, Used in Calculating Benefit Obligation + References No definition available. + Details
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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 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 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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