Operating Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Public Utilities, General Disclosures [Line Items]
Gas transportation		$ 6.1	$ 6.3	$ 19.3	$ 18.0
Other	[1]	6.9	6.0	23.5	16.3
Total operating revenue	[1]	90.9	87.9	627.5	624.0
Residential and commercial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial gas sales		75.3	72.1	575.8	576.7
Industrial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial gas sales		$ 2.6	$ 3.5	$ 8.9	$ 13.0

[1] See Note 12 for amounts attributable to related parties.