REGULATORY ASSETS AND LIABILITIES - Regulatory Assets (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets, current		$ 9.6	$ 69.8
Regulatory assets, noncurrent		5.5	11.9
Total regulatory assets		15.1	81.7
Regulatory assets that were not earning a return		2.6	20.3
Purchased-gas adjustment
Regulatory Assets [Line Items]
Regulatory assets, current	[1]	3.4	18.9
EEP
Regulatory Assets [Line Items]
Regulatory assets, current	[2]	1.1	1.1
Contract withholding
Regulatory Assets [Line Items]
Regulatory assets, current	[3]	2.6	20.3
Deferred cost-of-service gas charges
Regulatory Assets [Line Items]
Regulatory assets, current	[4]	0.0	19.5
Regulatory assets, noncurrent	[4]	0.0	8.1
Pipeline integrity costs
Regulatory Assets [Line Items]
Regulatory assets, current	[5]	1.9	6.3
Regulatory assets, noncurrent	[5]	2.3	0.0
Pipeline integrity costs | Maximum
Regulatory Assets [Line Items]
Allowed recovery amount per year		7.0
CET
Regulatory Assets [Line Items]
Regulatory assets, current	[6]	0.0	3.6
Other
Regulatory Assets [Line Items]
Regulatory assets, current		0.6	0.1
Cost of reacquired debt
Regulatory Assets [Line Items]
Regulatory assets, noncurrent	[7]	$ 3.2	$ 3.8
Cost of reacquired debt | Weighted Average
Regulatory Assets [Line Items]
Recovery period		6 years 1 month

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

[2] 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.

[3] In 2016, Questar Gas recorded a regulatory asset of $2.6 million for a disputed amount withheld from a supplier of storage services. The amount withheld is expected to be recovered from customers if it is determined that Questar Gas is required to pay the supplier. The $20.3 million withheld from a supplier of gathering services as of year-end 2015, per the dispute settlement agreement, was resolved and reversed in March 2016. For further details, see Note 16.

[4] Operating and maintenance, depreciation, depletion and amortization, production taxes and royalties on cost-of-service gas production and future expenses related to abandonment of Wexpro-operated gas and oil wells. Noncurrent cost-of-service gas charges also include amounts for production imbalances that will be recovered from customers at the end of the related gas wells' useful lives. These costs were transferred to Wexpro in September 2016.

[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] Represents the difference between actual and allowed revenues. Any deficiency in amounts collected are recovered through periodic rate adjustments.

[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 6.1 years as of December 31, 2016.