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Postretirement Health Care and Life Insurance Benefits Assumptions Used to Determine the Benefit Obligations and Periodic Benefit Cost (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 01, 2017 | Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2020 years | Dec. 31, 2019 years | Dec. 31, 2018 USD ($) | Dec. 31, 2017 years | Dec. 31, 2016
Postretirement Health Care and Life Insurance Benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Defined Benefit Plan, Amortization Period | years |  |  |  |  | 5.9 |  | 10.3 |
Weighted-average assumptions used to determine benefit obligations |  |  |  |  |  |  |  |
Discount rate |  |  |  |  | 3.40% | 4.35% |  |
Measurement date | Apr. 01, 2017 |  |  |  | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Weighted-average assumptions used to determine net periodic benefit cost |  |  |  |  |  |  |  |
Discount rate |  | 4.15% | 4.10% |  | 4.35% | 3.70% |  |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets |  | 0.00% | 0.00% |  | 5.00% | 0.00% |  |
Assumed health care cost trend rates, One percentage point increase |  |  |  |  |  |  |  |
One Percentage-Point Increase Effect on total service and interest cost components |  |  |  |  |  | $ 2.5 |  |
One Percentage-Point Decrease Effect on total service and interest cost component |  |  |  |  |  | (2.3) |  |
One Percentage-Point Increase Effect on total postretirement benefit obligation |  |  |  |  |  | 57.3 |  |
One Percentage-Point Decrease Effect on total postretirement benefit obligation |  |  |  |  |  | $ (50.9) |  |
Pre-Medicare |  |  |  |  |  |  |  |
Assumed health care cost trend rate |  |  |  |  |  |  |  |
Health care cost trend rate assumed for next year |  |  |  |  | 6.75% | 6.55% |  |
Rate to which the cost trend is assumed to decline (the ultimate trend rate) |  |  |  |  | 4.75% | 4.75% |  |
Year that the rate reaches the ultimate trend rate |  |  |  |  | 2023 |  | 2023 |
Post-Medicare |  |  |  |  |  |  |  |
Assumed health care cost trend rate |  |  |  |  |  |  |  |
Health care cost trend rate assumed for next year |  |  |  |  | 6.35% | 6.15% |  |
Rate to which the cost trend is assumed to decline (the ultimate trend rate) |  |  |  |  | 4.75% | 4.75% |  |
Year that the rate reaches the ultimate trend rate |  |  |  |  | 2023 |  | 2023 |
Scenario, Forecast | Postretirement Health Care and Life Insurance Benefits |  |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |
Defined Benefit Plan, Amortization Period | years |  |  |  | 4.9 |  |  |  |
Weighted-average assumptions used to determine net periodic benefit cost |  |  |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets |  |  |  | 7.00% |  |  |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=118255775&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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