Statements of Consolidated Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 4.7	$ 26.0	$ 40.7	$ 52.3
Other Comprehensive Income (Loss), Net of Tax - Note 13:
Defined benefit pension plan and VEBAs	0.8	0.7	1.7	1.5
Available for sale securities	0.2	0.3	0.3	0.3
Other	0.1	(0.1)	0.4	0.0
Other comprehensive income, net of tax	1.1	0.9	2.4	1.8
Comprehensive income	$ 5.8	$ 26.9	$ 43.1	$ 54.1