Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets-current		$ 13.4	$ 9.6	[1]
Regulatory assets-noncurrent	[2]	4.4	5.5
Total regulatory assets		17.8	15.1
Purchased-gas adjustment
Regulatory Assets [Line Items]
Regulatory assets-current	[3]	10.8	3.4
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]	1.1	2.3
Pipeline integrity costs | Maximum
Regulatory Assets [Line Items]
Allowed recovery amount per year		7.0
Other
Regulatory Assets [Line Items]
Regulatory assets-current		0.7	1.7
Regulatory assets-noncurrent		0.4
Cost of reacquired debt
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	[6]	$ 2.9	$ 3.2
Cost of reacquired debt | Weighted Average
Regulatory Assets [Line Items]
Recovery period		5 years 7 months 6 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] 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] 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 5.6 years as of June 30, 2017.