Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets-current		$ 1.9	$ 9.6	[1]
Regulatory assets-noncurrent		5.7	5.5
Total regulatory assets		7.6	15.1
Purchased-gas adjustment
Regulatory Assets [Line Items]
Regulatory assets-current	[2]	0.0	3.4
EEP
Regulatory Assets [Line Items]
Regulatory assets-current	[3]	0.0	1.1
Contract withholding
Regulatory Assets [Line Items]
Regulatory assets-current	[4]	0.0	2.6
Pipeline integrity costs
Regulatory Assets [Line Items]
Regulatory assets-current	[5]	1.9	1.9
Regulatory assets-noncurrent	[5]	1.4	2.3
Pipeline integrity costs | Maximum
Regulatory Assets [Line Items]
Allowed recovery amount per year		7.0
Cost of reacquired debt
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		$ 3.1	3.2
Cost of reacquired debt | Weighted Average
Regulatory Assets [Line Items]
Recovery period		5 years 9 months
Derivatives
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	[5]	$ 1.2	0.0
Other
Regulatory Assets [Line Items]
Regulatory assets-current		$ 0.0	$ 0.6

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

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

[3] 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 incurred result in a liability.

[4] 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.