STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME (LOSS) (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (6.6)	$ 19.2	$ 22.5	$ 47.2
Other comprehensive income (loss), net of tax
Defined benefit pension plan and Salaried VEBA	0.9	1.1	2.0	2.2
Available for sale securities	(0.3)	(0.1)	(0.5)	(0.3)
Cash flow hedges	10.8	(4.2)	(5.1)	4.6
Fair value hedges	0.5
Other comprehensive income (loss), net of tax	11.9	(3.2)	(3.6)	6.5
Comprehensive income	$ 5.3	$ 16.0	$ 18.9	$ 53.7