Operating Revenue - Subsequent 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
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		$ 82.3		$ 620.4
Other revenue	[1]	1.8		7.8
Total operating revenue	[1]	84.1	$ 90.9	628.2	$ 627.5
Regulated Gas Sales | Residential
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		60.9		448.9
Regulated Gas Sales | Commercial
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		8.7		127.8
Regulated Gas Sales | Industrial
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		0.9		4.3
Regulated Gas Transportation
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		6.5		20.2
Other Regulated Revenue
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		1.9		6.6
Other Nonregulated Revenue
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers	[1],[2]	$ 3.4		$ 12.6

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

[2] The amounts above include sales of oil and extracted products of $2.0 million and $7.7 million for the three and nine months ended September 30, 2018, respectively, which are considered to be goods transferred at a point in time.