Employee Benefits - Summary of Components of Net Periodic Postretirement Benefit Cost Relating to Salaried VEBA (Details) - Postretirement Health Coverage - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Salaried VEBA:
Interest cost	$ 0.7	$ 0.8	$ 1.3	$ 1.6
Expected return on plan assets	(0.7)	(0.7)	(1.4)	(1.3)
Amortization of prior service cost	1.1	1.4	2.3	2.8
Amortization of net actuarial loss	0.1	0.1	0.2	0.2
Salaried VEBA
Salaried VEBA:
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.2	$ 1.6	$ 2.4	$ 3.3