Statements of Income - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating revenue	[1]	$ 921.3	$ 917.6	$ 960.9
Operating Expenses
Cost of sales	[1]	542.1	558.1	604.8
Other operations and maintenance	[1]	176.6	162.5	175.2
Depreciation and amortization		61.0	55.1	53.6
Other taxes		20.9	19.3	17.8
Total Operating Expenses		800.6	795.0	851.4
Income from operations		120.7	122.6	109.5
Other income		3.8	4.8	5.9
Interest expense	[1]	30.2	28.3	28.2
Income from operations before income tax expense		94.3	99.1	87.2
Income tax expense		37.1	34.8	32.0
Net Income		$ 57.2	$ 64.3	$ 55.2

[1] See Note 18 for amounts attributable to related parties.