Operating Revenue (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Public Utilities, General Disclosures [Line Items]
Gas transportation		$ 6.3	$ 5.7	$ 13.2	$ 11.7
Other	[1]	8.3	5.1	16.6	10.3
Total operating revenue	[2]	139.7	128.2	536.6	536.1
Residential and commercial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		122.0	112.9	500.5	504.6
Industrial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		$ 3.1	$ 4.5	$ 6.3	$ 9.5

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

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