Statements of Income - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating Revenue	[1]	$ 84.1	$ 90.9	$ 628.2	$ 627.5
Operating Expenses
Purchased gas	[1]	35.9	42.3	361.5	359.1
Other operations and maintenance	[1]	31.7	30.2	112.2	116.5
Depreciation and amortization		19.0	19.4	56.3	52.1
Other taxes		6.3	5.6	19.0	17.0
Total Operating Expenses		92.9	97.5	549.0	544.7
Income (loss) from operations		(8.8)	(6.6)	79.2	82.8
Other income		0.8	1.1	2.6	3.1
Interest and related charges	[1]	8.9	8.5	27.0	25.5
Income (loss) from operations before income tax expense		(16.9)	(14.0)	54.8	60.4
Income tax expense (benefit)		(5.3)	(5.5)	10.1	22.9
Net Income (Loss)		$ (11.6)	$ (8.5)	$ 44.7	$ 37.5

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