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Pension and Other Postretirement Benefits - Weighted average assumptions used to determine net periodic benefit cost (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Rate of compensation increase, year one | 3.00% | 3.00% |
Rate of compensation increase, year one and thereafter | 3.50% | 3.50% |
Rate of compensation increase, year two |  |  | 3.00%
Rate of compensation increase, year two and thereafter |  |  | 4.00%
Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2019 | 12/31/2018 | 12/31/2017
Fiscal year end | 12/31/2020 | 12/31/2019 | 12/31/2018
Discount rate | 3.22% | 4.38% | 3.69%
Rate of compensation increase, group 1 | 3.00% | 3.00% | 3.00%
Rate of compensation increase, group 2 | 3.50% | 3.50% | 4.00%
Expected return on plan assets | 7.25% | 7.25% | 7.18%
OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] |  |  |
Measurement date | 12/31/2019 | 12/31/2018 | 12/31/2017
Fiscal year end | 12/31/2020 | 12/31/2019 | 12/31/2018
Discount rate | 2.79% | 4.29% | 3.66%
Rate of compensation increase, group 1 | 3.00% | 3.00% | 3.00%
Rate of compensation increase, group 2 | 3.50% | 3.50% | 4.00%
Expected return on plan assets | 0.00% | 0.00% | 0.00%

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- Definition The maximum expected rate of compensation increases (for pay-related plans). + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One And Thereafter + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Two + References No definition available. + Details
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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Two And Thereafter + References No definition available. + Details
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- Definition The date(s) used to measure (at least the majority of) plan assets and benefit obligations for defined benefit pension and other postretirement benefit plans. + 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 net periodic benefit cost of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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://www.xbrl.org/2003/role/disclosureRef -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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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