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Employee Benefits - Additional Information (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Payment for Other Postretirement Benefits |  | $ 1,700,000 | $ 2,900,000 |
Warrick | Accrued Salaries, Wages and Related Expenses |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | $ 1,100,000 |  |  |
Postretirement Health Coverage | Salaried VEBA |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Maximum contribution threshold |  |  |  | $ 2,900,000
Payment for Other Postretirement Benefits | 1,700,000 |  |  |
Pension Plan | Warrick |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | 6,500,000 |  |  |
Expected minimum employer contributions in 2021 | 100,000 |  |  |
Other Postretirement Benefits Plan | Warrick |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accumulated benefit obligation | 78,100,000 |  |  |
Expected minimum employer contributions in 2021 | $ 700,000 |  |  |
Level 2 | Fair Value, Measurements, Recurring |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of deferred compensation assets |  | 10,300,000 |  | $ 9,600,000
Accrued salaries, wages and related expenses |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Accrued salaries, wages and related expenses |  | $ 6,700,000 |  |

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