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