Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets-current		$ 47.3	$ 9.6	[1]
Regulatory assets-noncurrent	[2]	3.9	5.5
Total regulatory assets		51.2	15.1
Purchased-gas adjustment
Regulatory Assets [Line Items]
Regulatory assets-current	[3]	40.6	3.4
Contract withholding
Regulatory Assets [Line Items]
Regulatory assets-current	[4]	0.1	2.6
Pipeline integrity costs
Regulatory Assets [Line Items]
Regulatory assets-current	[5]	2.0	1.9
Regulatory assets-noncurrent	[5]	1.1	2.3
Pipeline integrity costs | Maximum
Regulatory Assets [Line Items]
Allowed recovery amount per year		7.0
EEP
Regulatory Assets [Line Items]
Regulatory assets-current	[6]	3.8	1.1
Other
Regulatory Assets [Line Items]
Regulatory assets-current		0.8	0.6
Cost of reacquired debt
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	[7]	$ 2.8	$ 3.2
Cost of reacquired debt | Weighted Average
Regulatory Assets [Line Items]
Recovery period		5 years 3 months 18 days

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

[2] Noncurrent regulatory assets are presented in other noncurrent assets in the Balance Sheets.

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

[4] The balance at December 31, 2016 represents a disputed amount withheld from a supplier of storage services. The dispute was settled in March 2017 and the amount reversed, which resulted in no material impact to Questar Gas' results of operations, financial position or cash flows.

[5] 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.

[6] 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.

[7] 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 5.3 years as of September 30, 2017.