Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost Relating to Salaried VEBA (Details) - Postretirement Health Coverage - Salaried VEBA - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Salaried VEBA:
Service cost	$ 0.1		$ 0.1
Interest cost	0.6	$ 0.8	1.9	$ 2.4
Expected return on plan assets	(0.7)	(0.7)	(2.1)	(2.0)
Amortization of prior service cost	1.2	1.4	3.5	4.2
Amortization of net actuarial loss		0.1	0.2	0.3
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.2	$ 1.6	$ 3.6	$ 4.9