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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Postretirement Benefits [Line Items] |  |  |
Projected benefit obligation | $ 7.9 | $ 8.4 |
Benefit obligation discount rate (as a percent) | 3.10% | 4.06% |
Components of net postretirement benefit expense |  |  |
Net postretirement benefit expense | $ 0.3 | $ 0.8 | $ 0.9

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