Statements of Income - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Operating Revenue	[1]	$ 947.0	$ 921.3	$ 917.6
Operating Expenses
Purchased gas	[1]	555.4	542.1	558.1
Other operations and maintenance	[1]	154.8	176.6	162.5
Depreciation and amortization		70.3	61.0	55.1
Other taxes		22.2	20.9	19.3
Total Operating Expenses		802.7	800.6	795.0
Income from operations		144.3	120.7	122.6
Other income		4.2	3.8	4.8
Interest and related charges	[1]	34.5	30.2	28.3
Income from operations before income tax expense		114.0	94.3	99.1
Income tax expense		46.5	37.1	34.8
Net Income		$ 67.5	$ 57.2	$ 64.3

[1] See Note 19 for amounts attributable to related parties.