Operating Revenue - Prior to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
(millions)
Residential, commercial, and industrial gas sales		$ 82.3		$ 620.4
Total operating revenue	[1]	84.1	$ 90.9	628.2	$ 627.5
Regulated Gas Sales | Industrial
(millions)
Residential, commercial, and industrial gas sales		0.9		4.3
Regulated Gas Transportation
(millions)
Residential, commercial, and industrial gas sales		$ 6.5		$ 20.2
Calculated under Revenue Guidance in Effect before Topic 606
(millions)
Other	[1]		6.9		23.5
Total operating revenue			90.9		627.5
Calculated under Revenue Guidance in Effect before Topic 606 | Regulated Gas Sales | Residential and commercial
(millions)
Residential, commercial, and industrial gas sales			75.3		575.8
Calculated under Revenue Guidance in Effect before Topic 606 | Regulated Gas Sales | Industrial
(millions)
Residential, commercial, and industrial gas sales			2.6		8.9
Calculated under Revenue Guidance in Effect before Topic 606 | Regulated Gas Transportation
(millions)
Residential, commercial, and industrial gas sales			$ 6.1		$ 19.3

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