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Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Net periodic postretirement benefit cost: |  |  |  |
Total net periodic postretirement benefit cost | $ 1.2 | $ 1.1 | $ 3.0 | $ 3.5
Postretirement Health Coverage | Salaried VEBA |  |  |  |
Net periodic postretirement benefit cost: |  |  |  |
Service cost |  | 0.1 |  | 0.1
Interest cost | 0.4 | 0.6 | 1.2 | 1.9
Expected return on plan assets | (0.8) | (0.7) | (2.4) | (2.1)
Amortization of prior service cost | 0.9 | 1.2 | 2.6 | 3.5
Amortization of net actuarial loss | 0.1 |  | 0.3 | 0.2
Total net periodic postretirement benefit cost | 0.6 | $ 1.2 | 1.7 | $ 3.6
Pension Plan | Warrick Rolling Mill |  |  |  |
Net periodic postretirement benefit cost: |  |  |  |
Service cost | 1.3 |  | 2.5 |
Interest cost |  |  | 0.1 |
Total net periodic postretirement benefit cost | 1.3 |  | 2.6 |
Other Postretirement Benefits Plan | Warrick Rolling Mill |  |  |  |
Net periodic postretirement benefit cost: |  |  |  |
Service cost | 0.3 |  | 0.7 |
Interest cost | 0.6 |  | 1.2 |
Total net periodic postretirement benefit cost | $ 0.9 |  | $ 1.9 |

X
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