Statements of Income - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating Revenue	[1]	$ 90.9	$ 87.9	$ 627.5	$ 624.0
Operating Expenses
Cost of sales	[1]	42.3	39.7	359.1	364.4
Other operations and maintenance	[1]	30.2	47.5	116.5	131.4
Depreciation and amortization		19.4	15.3	52.1	45.0
Other taxes		5.6	5.2	17.0	15.8
Total Operating Expenses		97.5	107.7	544.7	556.6
Income (loss) from operations		(6.6)	(19.8)	82.8	67.4
Other income		1.1	0.9	3.1	2.8
Interest expense	[1]	8.5	7.3	25.5	22.5
Income (loss) from operations before income tax expense		(14.0)	(26.2)	60.4	47.7
Income tax expense (benefit)		(5.5)	(8.5)	22.9	19.4
Net Income (Loss)		$ (8.5)	$ (17.7)	$ 37.5	$ 28.3

[1] See Note 12 for amounts attributable to related parties.