Statements of Consolidated Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 25.7	$ 36.0
Other Comprehensive Income (Loss), Net of Tax
Defined benefit pension plan and Salaried VEBA	1.2	0.9
Available for sale securities	(0.2)	0.1
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	(9.3)	0.3
Other comprehensive (loss) income, net of tax	(8.3)	1.3
Comprehensive income	$ 17.4	$ 37.3