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
	Mar. 31, 2021	Mar. 31, 2020
Salaried VEBA:
Interest cost	$ 0.4	$ 0.6
Expected return on plan assets	(0.8)	(0.7)
Amortization of prior service cost	0.9	1.2
Amortization of net actuarial loss	0.1	0.1
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 0.6	$ 1.2