STATEMENTS OF CONSOLIDATED COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 28.8	$ 62.0	$ 91.7
Other comprehensive income (loss), net of tax
Defined benefit pension plan and Salaried VEBA	13.4	2.4	2.9
Available for sale securities	(0.3)		(0.6)
Cash flow hedges	6.7	7.8	(13.9)
Foreign currency translation	0.1		(0.1)
Other comprehensive income (loss), net of tax	19.9	10.2	(11.7)
Comprehensive income	$ 48.7	$ 72.2	$ 80.0