STATEMENTS OF CONSOLIDATED COMPREHENSIVE (LOSS) INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (22.4)	$ (6.6)	$ (17.9)	$ 22.5
Other comprehensive income (loss), net of tax
Defined benefit pension plan and Salaried VEBA	0.7	0.9	1.4	2.0
Available for sale securities		(0.3)		(0.5)
Cash flow hedges	15.2	10.8	23.7	(5.1)
Fair value hedges		0.5
Foreign currency translation			(0.1)
Other comprehensive income (loss), net of tax	15.9	11.9	25.0	(3.6)
Comprehensive (loss) income	$ (6.5)	$ 5.3	$ 7.1	$ 18.9