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Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | $ 1.2 | $ 1.2 | $ 1.8 | $ 2.4
Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined contribution plan, cost | 3.6 | 1.7 | 7.0 | 5.6
Deferred compensation arrangement with individual, compensation expense | 0.4 | 0.8 | 0.4 | 0.4
Multiemployer plan, contributions by employer | 1.3 | 1.2 | 2.4 | 2.5
Total other employee benefit plans | 8.1 | 4.9 | 13.2 | 10.9
Other Postretirement Benefits Plan | Warrick Rolling Mill |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | 1.0 |  | 1.0 |
Postretirement Health Coverage | Salaried VEBA |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | 0.5 | $ 1.2 | 1.1 | $ 2.4
Pension Plan | Warrick Rolling Mill |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total net periodic postretirement benefit cost | $ 1.3 |  | $ 1.3 |

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