Operating Revenue - Subsequent to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		$ 404.2
Other revenue		2.5
Total operating revenue	[1]	406.7	$ 396.9
Regulated Gas Sales | Residential
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		299.3
Regulated Gas Sales | Commercial
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		86.7
Regulated Gas Sales | Industrial
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		2.0
Regulated Gas Transportation
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		7.5
Other Regulated Revenue
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		3.8
Other Nonregulated Revenue
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers	[1]	$ 4.9

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