Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net periodic postretirement benefit cost:
Total net periodic postretirement benefit cost	$ 1.2	$ 1.1	$ 3.0	$ 3.5
Postretirement Health Coverage | Salaried VEBA
Net periodic postretirement benefit cost:
Service cost		0.1		0.1
Interest cost	0.4	0.6	1.2	1.9
Expected return on plan assets	(0.8)	(0.7)	(2.4)	(2.1)
Amortization of prior service cost	0.9	1.2	2.6	3.5
Amortization of net actuarial loss	0.1		0.3	0.2
Total net periodic postretirement benefit cost	0.6	$ 1.2	1.7	$ 3.6
Pension Plan | Warrick Rolling Mill
Net periodic postretirement benefit cost:
Service cost	1.3		2.5
Interest cost			0.1
Total net periodic postretirement benefit cost	1.3		2.6
Other Postretirement Benefits Plan | Warrick Rolling Mill
Net periodic postretirement benefit cost:
Service cost	0.3		0.7
Interest cost	0.6		1.2
Total net periodic postretirement benefit cost	$ 0.9		$ 1.9