Employee Benefits - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Payment for Other Postretirement Benefits	$ 1,700,000	$ 2,900,000
Warrick Rolling Mill | Other Accrued Liabilities
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,000,000.0
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 Rolling Mill
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	6,700,000
Expected minimum employer contributions in 2021	100,000
Other Postretirement Benefits Plan | Warrick Rolling Mill
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	78,000,000.0
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	9,500,000		$ 9,600,000
Accrued salaries, wages and related expenses
Defined Benefit Plan Disclosure [Line Items]
Accrued salaries, wages and related expenses	$ 2,700,000