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Benefit Plans and Other Postretirement Benefits, OPEB benefit obligation and benefit expense (Details) - Other Postretirement Benefits - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Postretirement Benefits [Line Items] |  |  |
Projected benefit obligation | $ 6.8 | $ 7.9 |
Benefit obligation discount rate (as a percent) | 2.40% | 3.10% |
Components of net postretirement benefit expense |  |  |
Net postretirement benefit expense | $ 0.2 | $ 0.3 | $ 0.8

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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/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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