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Pension and Other Postretirement Benefits - Changes in Plan Assets & Benefit Obligation Recognized in OCI (pre-taxed) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net loss (gain) | $ 10.1 | $ 5.9 | $ (31.6)
Amortization of prior service (cost) benefit, including curtailment | (0.1) | 1.2 | 3.1
Pension |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net loss (gain) | 4.4 | 29.0 |
Prior service cost (benefit) | 1.2 | 0.0 |
Amortization of net loss, including recognition due to settlement | (6.0) | (3.5) |
Amortization of prior service (cost) benefit, including curtailment | (0.1) | (0.1) |
Total amount recognized in other comprehensive loss | (0.5) | 25.4 |
Total benefit cost | 7.4 | (5.3) | (1.9)
Total recognized in net periodic benefit cost and other comprehensive loss | 6.9 | 20.1 |
OPEB |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net loss (gain) | 4.5 | (23.1) |
Prior service cost (benefit) | 0.0 | 0.0 |
Amortization of net loss, including recognition due to settlement | (0.2) | (1.4) |
Amortization of prior service (cost) benefit, including curtailment | 0.0 | 10.3 |
Total amount recognized in other comprehensive loss | 4.3 | (14.2) |
Total benefit cost | 4.1 | (5.8) | $ 1.7
Total recognized in net periodic benefit cost and other comprehensive loss | $ 8.4 | $ (20.0) |

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