Related-Party Transactions (Details) - USD ($)		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Receivable from affiliates		$ 100,300,000		$ 87,800,000	[1]
Payables to affiliates		13,100,000		13,000,000	[1]
Affiliated current borrowings		0		$ 48,000,000	[1]
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	75,900,000	$ 79,800,000
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	19,600,000	19,200,000
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		10,800,000	13,500,000
Related party revenues		1,500,000	500,000
Dominion | Revolving Credit Facility | IRCA
Related Party Transaction [Line Items]
Weighted-average interest rate percentage				1.04%
Related party interest expense		0	300,000
Questar Pipeline | Transportation services
Related Party Transaction [Line Items]
Annual commitment		55,000,000
Questar Pipeline | Storage contract
Related Party Transaction [Line Items]
Annual commitment		2,000,000
Dominion Questar Corporation | Dominion Questar Corporation | Merger and restructuring costs
Related Party Transaction [Line Items]
Transaction costs		$ 300,000	$ 0

[1] Questar Gas' Balance Sheet at December 31, 2016 has been derived from the audited Balance Sheet at that date.

[2] The costs of these services were included in cost of sales in Questar Gas' Statements of Income.