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Employee Benefits - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2021 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Payment for Other Postretirement Benefits |  | $ 1.7 | $ 2.9 |
Warrick | Accrued Salaries, Wages and Related Expenses |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | $ 1.1 |  |  |
Postretirement Health Coverage | Salaried VEBA |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Maximum contribution threshold |  |  |  | $ 2.9
Payment for Other Postretirement Benefits | 1.7 |  |  |
Pension Plan | Warrick |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | 6.5 |  |  |
Expected minimum employer contributions in 2021 | 0.1 |  |  |
Other Postretirement Benefits Plan | Warrick |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | 78.1 |  |  |
Expected minimum employer contributions in 2021 | $ 0.7 |  |  |
Level 2 | Fair Value, Measurements, Recurring |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of deferred compensation assets |  | 10.4 |  | $ 9.6
Accrued salaries, wages and related expenses |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accrued salaries, wages and related expenses |  | $ 7.3 |  |

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