Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Total net periodic postretirement benefit cost	$ 1.2	$ 1.1	$ 3.0	$ 3.5
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, cost	3.4	1.4	10.4	7.0
Deferred compensation arrangement with individual, compensation expense	0.2	0.4	0.6	0.8
Multiemployer plan, contributions by employer	1.3	1.3	3.7	3.8
Total other employee benefit plans	7.7	4.3	20.9	15.2
Other Postretirement Benefits Plan | Warrick Rolling Mill
Defined Benefit Plan Disclosure [Line Items]
Total net periodic postretirement benefit cost	0.9		1.9
Postretirement Health Coverage | Salaried VEBA
Defined Benefit Plan Disclosure [Line Items]
Total net periodic postretirement benefit cost	0.6	$ 1.2	1.7	$ 3.6
Pension Plan | Warrick Rolling Mill
Defined Benefit Plan Disclosure [Line Items]
Total net periodic postretirement benefit cost	$ 1.3		$ 2.6