Other Income (Expense), Net (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Income And Expenses [Abstract]
Interest income	$ 0.2	$ 0.1	$ 0.3	$ 0.3
Net periodic postretirement benefit cost relating to Salaried VEBA	(1.2)	(1.6)	(2.4)	(3.3)
Realized gain on available for sale securities	0.6	1.2	2.1	2.4
Unrealized gain on equity securities	0.7		0.1	0.5
All other income (expense), net	0.2	0.2	(0.4)	0.5
Other income (expense), net	$ 0.5	$ (0.1)	$ (0.3)	$ 0.4