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Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | 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) |  |  | 5.00% | 5.00%
Ultimate health care cost trend rate (as a percent) |  |  | 5.00% | 5.00%
Number of years to ultimate trend rate (pre-65 participants) |  |  | 4 years | 4 years
Number of years to ultimate trend rate (post-65 participants) |  |  | 0 years | 0 years
Weighted-average assumptions used to determine net periodic benefit costs |  |  |  |
Initial pre-65 health care cost trend rate (as a percent) | 7.50% | 7.50% | 7.00% | 7.00%
Initial post-65 health care cost trend rate (as a percent) | 5.00% | 7.50% | 5.00% | 5.00%
Ultimate healthcare cost trend rate (as a percent) | 5.00% | 5.00% | 5.00% | 5.00%
Number of years to ultimate trend rate (pre-65 participants) | 4 years | 4 years | 4 years | 4 years
Number of years to ultimate trend rate (post-65 participants) | 0 years | 4 years | 0 years | 0 years
Pension Benefits |  |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |  |
Discount rate (as a percent) |  |  | 4.08% | 4.37%
Weighted-average assumptions used to determine net periodic benefit costs |  |  |  |
Discount rate (as a percent) | 4.88% | 4.88% | 4.37% | 4.02%
Rate of compensation increase (as a percent) | 4.00% | 4.00% | 4.00% | 4.00%
Expected long-term return on plan assets (as a percent) | 6.90% | 6.90% | 6.90% | 6.90%
Other Benefits |  |  |  |
Weighted-average assumptions used to determine benefit obligations |  |  |  |
Discount rate (as a percent) |  |  | 4.17% | 4.52%
Weighted-average assumptions used to determine net periodic benefit costs |  |  |  |
Discount rate (as a percent) | 4.41% | 5.10% | 4.52% | 4.14%
Expected long-term return on plan assets (as a percent) | 4.25% | 6.80% | 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 |  |  | $ 8,430 |
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants |  |  | (5,455) |
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs |  |  | 8,440 |
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs |  |  | (6,527) |
Effect of 1% increase on the accumulated other postretirement benefit obligation |  |  | 108,046 |
Effect of 1% decrease on the accumulated other postretirement benefit obligation |  |  | $ (86,651) |

X
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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, Pre-65 Participant, Period after which Rate Reaches Ultimate Trend Rate, used in Calculating Benefit Costs + References No definition available. + Details
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- Definition The interest rate used to adjust for the time value of money for the 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=65897934&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + 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=65897934&loc=d3e1928-114920 + Details
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- Definition The interest rate used to adjust for the time value of money. + 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=65897934&loc=d3e1928-114920 + Details
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512046 Reference 2: 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=65897934&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + 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=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + 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=65897934&loc=d3e1928-114920 + Details
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