Number Of Former Manufactured Gas Plant Sites

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COMMITMENTS AND CONTINGENCIES (Details) $ in Millions	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) $ in Millions	Dec. 31, 2016 USD ($) site	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Purchase Obligation, Fiscal Year Maturity [Abstract]
Gas purchases	$ 102.0	$ 82.5	$ 136.5
Transportation and Underground Storage Services with Affiliated Entity \| Questar Pipeline \| Questar Pipeline, LLC
Contractual Obligation, Fiscal Year Maturity [Abstract]
2017	44.0
2018	13.0
2019	3.8
2020	1.9
2021	1.6
Commitments to Purchase Gas
Purchase Obligation, Fiscal Year Maturity [Abstract]
2017	29.3
2018	25.2
2019	25.2
2020	25.4
2021	25.2
Gas purchases	102.0	$ 82.4	$ 135.8
Transportation and Gathering Commitments
Other Commitment, Fiscal Year Maturity [Abstract]
2017	27.8
2018	24.4
2019	22.7
2020	22.7
2021	$ 19.6
Unfavorable Regulatory Action \| Coal Tar and Other Potentially Harmful Materials \| EPA
Other Commitments [Line Items]
Number of former manufactured gas plant sites \| site	2