STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 0.4	$ 25.4	$ 22.9	$ 72.6
Other comprehensive income (loss), net of tax
Defined benefit pension plan and Salaried VEBA	0.8	1.2	2.8	3.4
Available for sale securities		0.1	(0.5)	(0.2)
Cash flow hedges	7.3	(2.2)	2.2	2.4
Foreign currency translation	0.1		0.1
Other comprehensive income (loss), net of tax	8.2	(0.9)	4.6	5.6
Comprehensive income	$ 8.6	$ 24.5	$ 27.5	$ 78.2