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Employee Benefits - Assumptions (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Pension Plan | Benefit obligation assumptions |  |  |  |
Weighted average assumptions - benefit obligations |  |  |  |
Discount rate |  | 3.41% | 4.30% |
Rate of compensation increase |  | 4.00% | 3.66% |
Pension Plan | Net periodic benefit cost assumptions |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Discount rate |  | 4.21% | 3.71% | 4.11%
Expected return on plan assets |  | 7.12% | 7.12% | 7.11%
Rate of compensation increase |  | 3.66% | 3.66% | 3.60%
Pension Plan | Net periodic benefit cost assumptions | Subsequent event |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Expected return on plan assets | 6.87% |  |  |
OPEB Plan | Benefit obligation assumptions |  |  |  |
Weighted average assumptions - benefit obligations |  |  |  |
Discount rate |  | 3.39% | 4.27% |
Effects of a one-percentage-point change in assumed health care cost trend rates |  |  |  |
Effect of one-percentage-point increase on the health care component of the accumulated postretirement benefit obligation |  | $ 43.5 |  |
Effect of one-percentage-point decrease on the health care component of the accumulated postretirement benefit obligation |  | $ (36.5) |  |
OPEB Plan | Benefit obligation assumptions | Pre 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 6.00% | 6.25% |
Ultimate trend rate |  | 5.00% | 5.00% |
Year ultimate trend rate is reached |  | 2028 | 2024 |
OPEB Plan | Benefit obligation assumptions | Post 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 5.91% | 6.01% |
Ultimate trend rate |  | 5.00% | 5.00% |
Year ultimate trend rate is reached |  | 2028 | 2028 |
OPEB Plan | Net periodic benefit cost assumptions |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Discount rate |  | 4.27% | 3.63% | 4.04%
Expected return on plan assets |  | 7.25% | 7.25% | 7.25%
Effects of a one-percentage-point change in assumed health care cost trend rates |  |  |  |
Effect of one-percentage-point increase on total of service and interest cost components of net periodic postretirement health care benefit cost |  | $ 4.7 |  |
Effect of one-percentage-point decrease on total of service and interest cost components of net periodic postretirement health care benefit cost |  | $ (3.8) |  |
OPEB Plan | Net periodic benefit cost assumptions | Subsequent event |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Expected return on plan assets | 7.00% |  |  |
OPEB Plan | Net periodic benefit cost assumptions | Pre 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 6.25% | 6.50% | 7.00%
Ultimate trend rate |  | 5.00% | 5.00% | 5.00%
Year ultimate trend rate is reached |  | 2024 | 2024 | 2021
OPEB Plan | Net periodic benefit cost assumptions | Post 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 6.01% | 6.09% | 7.00%
Ultimate trend rate |  | 5.00% | 5.00% | 5.00%
Year ultimate trend rate is reached |  | 2028 | 2028 | 2021

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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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