Operating Revenue (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Public Utilities, General Disclosures [Line Items]
Gas transportation		$ 6.9	$ 6.0
Other	[1]	8.3	5.2
Total operating revenue	[2]	396.9	407.9
Residential and commercial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		378.5	391.7
Industrial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		$ 3.2	$ 5.0

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

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