Operating Revenue - Narrative (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	[1]
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		$ 82.3	$ 620.4
Contract liability balances		17.8	17.8	$ 17.3
Contract liability, revenue recognized			17.3
Other Nonregulated Revenue
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers	[2],[3]	3.4	12.6
Other Nonregulated Revenue | Transferred at Point in Time
Public Utilities General Disclosures [Line Items]
Total operating revenue from contracts with customers		$ 2.0	$ 7.7

[1] Questar Gas’ Balance Sheet at December 31, 2017 has been derived from the audited Balance Sheet at that date.

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

[3] 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.