Statements of Income - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating Revenue	[1]	$ 406.7	$ 396.9
Operating Expenses
Purchased gas	[1]	251.6	241.3
Other operations and maintenance	[1]	45.0	41.7
Depreciation and amortization		18.6	16.2
Other taxes		6.4	5.8
Total Operating Expenses		321.6	305.0
Income from operations		85.1	91.9
Other income		0.8	1.0
Interest and related charges	[1]	9.2	8.5
Income from operations before income tax expense		76.7	84.4
Income tax expense		17.6	32.2
Net Income		$ 59.1	$ 52.2

[1] See Note 13 for amounts attributable to related parties.