Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	As permitted by the rules and regulations of the SEC, Questar Gas' accompanying unaudited Financial Statements contain certain condensed financial information and exclude certain footnote disclosures normally included in annual audited financial statements prepared in accordance with GAAP.
Use of Estimates	Questar Gas makes certain estimates and assumptions in preparing its Financial Statements in accordance with GAAP. These estimates and assumptions affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues, expenses and cash flows for the periods presented. Actual results may differ from those estimates.
Fair Value Measurements	Questar Gas reports certain contracts and instruments at fair value. Questar Gas enters into certain physical forwards and options, which are considered Level 3 as they have one or more inputs that are not observable and are significant to the valuation. The discounted cash flow method is used to value Level 3 physical forward contracts. An option model is used to value Level 3 physical options. The discounted cash flow model for forwards calculates mark-to-market valuations based on forward market prices, original transaction prices, volumes, risk-free rate of return, and credit spreads. The option model calculates mark-to-market valuations using variations of the Black-Scholes option model. The inputs into the models are the forward market prices, implied price volatilities, risk-free rate of return, the option expiration dates, the option strike prices, the original sales prices, and volumes. For Level 3 fair value measurements, forward market prices and implied price volatilities are considered unobservable. The unobservable inputs are developed and substantiated using historical information, available market data, third-party data, and statistical analysis. Periodically, inputs to valuation models are reviewed and revised as needed, based on historical information, updated market data, market liquidity and relationships, and changes in third-party pricing sources.
Reclassifications	Certain amounts in the 2016 Financial Statements and Notes have been reclassified to conform to the 2017 presentation for comparative purposes. The reclassifications did not affect Questar Gas' net income, total assets, liabilities, equity or cash flows
Derivatives	Derivative assets and liabilities are presented gross on Questar Gas' Balance Sheets. Questar Gas' derivative contracts include over-the-counter transactions, which are bilateral contracts that are transacted directly with a counterparty. At September 30, 2017, substantially all of Questar Gas' derivative assets and liabilities were not subject to a master netting or similar arrangement.
Commitments and Contingencies	As a result of issues generated in the ordinary course of business, Questar Gas is involved in legal proceedings before various courts and is periodically subject to governmental examinations (including by regulatory authorities), inquiries and investigations. Certain legal proceedings and governmental examinations involve demands for unspecified amounts of damages, are in an initial procedural phase, involve uncertainty as to the outcome of pending appeals or motions, or involve significant factual issues that need to be resolved, such that it is not possible for Questar Gas to estimate a range of possible loss. For such matters for which Questar Gas cannot estimate a range of possible loss, a statement to this effect is made in the description of the matter. Other matters may have progressed sufficiently through the litigation or investigative processes such that Questar Gas is able to estimate a range of possible loss. For legal proceedings and governmental examinations for which Questar Gas is able to reasonably estimate a range of possible losses, an estimated range of possible loss is provided, in excess of the accrued liability (if any) for such matters. Any accrued liability is recorded on a gross basis with a receivable also recorded for any probable insurance recoveries. Estimated ranges of loss are inclusive of legal fees and net of any anticipated insurance recoveries. Any estimated range is based on currently available information and involves elements of judgment and significant uncertainties. Any estimated range of possible loss may not represent Questar Gas' maximum possible loss exposure. The circumstances of such legal proceedings and governmental examinations will change from time to time and actual results may vary significantly from the current estimate.
Credit Risk	Questar Gas maintains a provision for credit losses based on factors surrounding the credit risk of its customers, historical trends and other information. Management believes, based on credit policies and the September 30, 2017 provision for credit losses, that it is unlikely that a material adverse effect on financial position, results of operations or cash flows would occur as a result of counterparty nonperformance.