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Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 14.3 | $ 17.4 | $ 26.1 |
Other Postretirement Benefits Plan | U.S. equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 3.0 | 3.7 |  |
Other Postretirement Benefits Plan | International equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.9 | 1.1 |  |
Other Postretirement Benefits Plan | Corporate bonds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 5.2 | 7.1 |  |
Other Postretirement Benefits Plan | U.S. government securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 4.6 | 5.0 |  |
Other Postretirement Benefits Plan | Cash funds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.6 | 0.5 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 6.3 | 7.3 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | U.S. equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 3.0 | 3.7 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | International equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.9 | 1.1 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | Corporate bonds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | U.S. government securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 1.8 | 2.0 |  |
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | Cash funds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.6 | 0.5 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 8.0 | 10.1 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | U.S. equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | International equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | Corporate bonds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 5.2 | 7.1 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | U.S. government securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 2.8 | 3.0 |  |
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | Cash funds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 430.4 | $ 0.3 | $ 1.2
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | U.S. equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | International equity securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | Corporate bonds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | U.S. government securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | Cash funds |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 0.0 | $ 0.0 |  |

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