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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax | $ (28.6) | $ (28.8) | $ (32.1) | $ (27.9)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax | 0.1 | (1.3) |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.3 | 0.4 |  |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | (30.4) | (34.3) | (34.1) | (35.1)
Other Comprehensive Income (Loss), before Reclassifications, before Tax | 0.0 | 0.0 |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.6 | 0.8 |  |
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax | 3.0 | 7.4 | 3.3 | 7.8
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax | 0.0 | 0.0 |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | (0.3) | (0.4) |  |
Commodity Contract [Member] |  |  |  |
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax | 0.1 | (1.3) |  |
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent | (1.2) | (1.9) | $ (1.3) | $ (0.6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 | $ 0.0 |  |
Settlement Cost [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 3.1 |  |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 3.1 |  |  |
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | 0.0 |  |  |
Settlement Cost [Member] | Commodity Contract [Member] |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax | $ 0.0 |  |  |

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