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Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total net periodic postretirement benefit cost | $ 8.4 | $ 5.6 | $ 4.1
Salaried VEBA |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service cost |  | 0.1 |
Interest cost | 2.9 | 1.9 |
Pension Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service cost | 3.8 | 5.8 | 4.1
Interest cost | 1.3 | 0.6 | 0.5
Expected return on plan assets | (1.1) | (0.9) | (0.4)
Amortization of prior service cost | 0.4 |  |
Amortization of net actuarial loss (gain) |  |  | 0.1
Total net periodic postretirement benefit cost | 4.4 | 5.5 | 4.3
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service cost | 1.1 | 1.6 | 1.1
Interest cost | 3.4 | 2.2 | 1.7
Amortization of net actuarial loss (gain) | (1.1) |  |
Total net periodic postretirement benefit cost | 3.4 | 3.8 | 2.8
Postretirement Health Coverage | Salaried VEBA |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Service cost |  | 0.1 | 0.1
Interest cost | 2.9 | 1.9 | 1.5
Expected return on plan assets | (2.2) | (3.1) | (3.1)
Amortization of prior service cost | 4.9 | 4.9 | 3.5
Amortization of net actuarial loss (gain) |  |  | 0.3
Total net periodic postretirement benefit cost | $ 5.6 | $ 3.8 | $ 2.3

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