Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($)		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets-current		$ 5,500,000	$ 16,600,000	[1]
Regulatory assets-noncurrent	[2]	7,800,000	3,300,000
Total regulatory assets		$ 13,300,000	19,900,000
Recovery period		4 years 7 months 6 days
Maximum
Regulatory Assets [Line Items]
Allowed recovery amount per year		$ 7,000,000
Pipeline integrity costs
Regulatory Assets [Line Items]
Regulatory assets-current	[3]	2,000,000	2,000,000
Regulatory assets-noncurrent	[3]	600,000	600,000
EEP
Regulatory Assets [Line Items]
Regulatory assets-current	[4]	1,900,000	3,300,000
Deferred royalties
Regulatory Assets [Line Items]
Regulatory assets-current	[5]	1,600,000
Purchased-gas adjustment
Regulatory Assets [Line Items]
Regulatory assets-current	[6]		10,700,000
Other
Regulatory Assets [Line Items]
Regulatory assets-current			600,000
Deferred production imbalance
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	[7]	4,700,000
Cost of reacquired debt
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	[8]	$ 2,500,000	$ 2,700,000

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

[2] Noncurrent regulatory assets are presented in the other deferred charges and other assets in the Balance Sheets.

[3] The costs of complying with pipeline-integrity regulations are recovered in rates subject to a Utah Commission order. Questar Gas is allowed to recover $7.0 million per year. Costs incurred in excess of this amount will be recovered in future rate changes.

[4] The EEP relates to funds expended for promoting the conservation of natural gas through advertising, rebates for efficient homes and appliances and home energy audits. Costs are recovered from customers through periodic rate adjustments. Costs incurred in excess of recoveries result in an asset; recoveries in excess of costs result in a liability.

[5] Royalties on cost-of-service gas produced are recovered from customers through future rates.

[6] Purchased gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes.

[7] Production imbalances will be recovered from customers at the end of the related gas wells’ useful life.

[8] Gains and losses on the reacquisition of debt by rate-regulated companies are deferred and amortized as interest expense over the would-be remaining life of the reacquired debt. The reacquired debt costs had a weighted-average life of approximately 4.6 years as of June 30, 2018.