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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | $ 1.2 | $ 1.1 | $ 3.0 | $ 3.5
Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, cost | 3.4 | 1.4 | 10.4 | 7.0
Deferred compensation arrangement with individual, compensation expense | 0.2 | 0.4 | 0.6 | 0.8
Multiemployer plan, contributions by employer | 1.3 | 1.3 | 3.7 | 3.8
Total other employee benefit plans | 7.7 | 4.3 | 20.9 | 15.2
Other Postretirement Benefits Plan | Warrick Rolling Mill |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | 0.9 |  | 1.9 |
Postretirement Health Coverage | Salaried VEBA |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | 0.6 | $ 1.2 | 1.7 | $ 3.6
Pension Plan | Warrick Rolling Mill |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | $ 1.3 |  | $ 2.6 |

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