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Employee Benefits - Assumptions (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Plan | Benefit obligation assumptions |  |  |  |
Weighted average assumptions - benefit obligations |  |  |  |
Discount rate |  | 3.66% | 4.16% |
Rate of compensation increase |  | 3.61% | 3.60% |
Pension Plan | Net periodic benefit cost assumptions |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Discount rate |  | 4.11% | 4.35% | 4.11%
Expected return on plan assets |  | 7.11% | 7.12% | 7.37%
Rate of compensation increase |  | 3.60% | 3.75% | 4.00%
Pension Plan | Net periodic benefit cost assumptions | Subsequent event |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Expected return on plan assets | 7.12% |  |  |
OPEB Plan | Benefit obligation assumptions |  |  |  |
Weighted average assumptions - benefit obligations |  |  |  |
Discount rate |  | 3.63% | 4.14% |
OPEB Plan | Benefit obligation assumptions | Pre 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 6.50% | 7.00% |
Ultimate trend rate |  | 5.00% | 5.00% |
Year ultimate trend rate is reached |  | 2024 | 2021 |
OPEB Plan | Benefit obligation assumptions | Post 65 |  |  |  |
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 6.09% | 7.00% |
Ultimate trend rate |  | 5.00% | 5.00% |
Year ultimate trend rate is reached |  | 2028 | 2021 |
OPEB Plan | Net periodic benefit cost assumptions |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Discount rate |  | 4.04% | 4.38% | 4.09%
Expected return on plan assets |  | 7.25% | 7.25% | 7.54%
Medical cost trend rates |  |  |  |
Assumed medical cost trend rate |  | 7.00% | 7.50% | 7.50%
Ultimate trend rate |  | 5.00% | 5.00% | 5.00%
Year ultimate trend rate is reached |  | 2021 | 2021 | 2021
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 |  | $ 8.0 |  |
Effect of one-percentage-point increase on the health care component of the accumulated postretirement benefit obligation |  | 76.2 |  |
Effect of one-percentage-point decrease on total of service and interest cost components of net periodic postretirement health care benefit cost |  | (6.4) |  |
Effect of one-percentage-point decrease on the health care component of the accumulated postretirement benefit obligation |  | $ (62.5) |  |
OPEB Plan | Net periodic benefit cost assumptions | Subsequent event |  |  |  |
Weighted average assumptions - net periodic benefit cost |  |  |  |
Expected return on plan assets | 7.25% |  |  |

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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 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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