Statements of Consolidated Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 19.9	$ 14.9	$ 60.6	$ 67.2
Other Comprehensive Income (Loss), Net of Tax - Note 13:
Salaried VEBA and defined benefit pension plan	0.8	0.7	2.5	2.2
Available for sale securities	0.3	0.3	0.6	0.6
Other	0.5	0.1	0.9	0.1
Other comprehensive income, net of tax	1.6	1.1	4.0	2.9
Comprehensive income	$ 21.5	$ 16.0	$ 64.6	$ 70.1