Statements of Income - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Operating revenue	[1]	$ 396.9	$ 407.9
Operating Expenses
Cost of sales	[1]	241.3	256.9
Other operations and maintenance	[1]	41.7	47.6
Depreciation and amortization		16.2	14.7
Other taxes		5.8	5.3
Total Operating Expenses		305.0	324.5
Income from operations		91.9	83.4
Other income		1.0	1.0
Interest expense	[1]	8.5	7.7
Income from operations before income tax expense		84.4	76.7
Income tax expense		32.2	29.1
Net Income		$ 52.2	$ 47.6

[1] See Note 11 for amounts attributable to related parties.