3.10.0.1 html 176 176 1 true 74 0 false 10 false false R1.htm 100000 - Document - Document and Entity Information Sheet http://www.questargas.com/20180930/taxonomy/role/DocumentDocumentAndEntityInformation Document and Entity Information Cover 1 false false R2.htm 100010 - Statement - Statements of Income Sheet http://www.questargas.com/20180930/taxonomy/role/StatementStatementsOfIncome Statements of Income Statements 2 false false R3.htm 100020 - Statement - Balance Sheets Sheet http://www.questargas.com/20180930/taxonomy/role/StatementBalanceSheets Balance Sheets Statements 3 false false R4.htm 100030 - Statement - Balance Sheets (Parenthetical) Sheet http://www.questargas.com/20180930/taxonomy/role/StatementBalanceSheetsParenthetical Balance Sheets (Parenthetical) Statements 4 false false R5.htm 100040 - Statement - Statements of Cash Flows Sheet http://www.questargas.com/20180930/taxonomy/role/StatementStatementsOfCashFlows Statements of Cash Flows Statements 5 false false R6.htm 100050 - Disclosure - Nature of Operations Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureNatureOfOperations Nature of Operations Notes 6 false false R7.htm 100060 - Disclosure - Significant Accounting Policies Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantAccountingPolicies Significant Accounting Policies Notes 7 false false R8.htm 100070 - Disclosure - Operating Revenue Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenue Operating Revenue Notes 8 false false R9.htm 100080 - Disclosure - Income Taxes Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureIncomeTaxes Income Taxes Notes 9 false false R10.htm 100090 - Disclosure - Fair Value Measurements Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurements Fair Value Measurements Notes 10 false false R11.htm 100100 - Disclosure - Derivatives and Hedge Accounting Activities Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivities Derivatives and Hedge Accounting Activities Notes 11 false false R12.htm 100110 - Disclosure - Regulatory Assets and Liabilities Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryAssetsAndLiabilities Regulatory Assets and Liabilities Notes 12 false false R13.htm 100120 - Disclosure - Regulatory Matters Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryMatters Regulatory Matters Notes 13 false false R14.htm 100130 - Disclosure - Variable Interest Entities Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureVariableInterestEntities Variable Interest Entities Notes 14 false false R15.htm 100140 - Disclosure - Significant Financing Transactions Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantFinancingTransactions Significant Financing Transactions Notes 15 false false R16.htm 100150 - Disclosure - Commitments and Contingencies Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureCommitmentsAndContingencies Commitments and Contingencies Notes 16 false false R17.htm 100160 - Disclosure - Credit Risk Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureCreditRisk Credit Risk Notes 17 false false R18.htm 100170 - Disclosure - Related-Party Transactions Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRelatedPartyTransactions Related-Party Transactions Notes 18 false false R19.htm 100180 - Disclosure - Operating Segment Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingSegment Operating Segment Notes 19 false false R20.htm 100190 - Disclosure - Significant Accounting Policies (Policies) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantAccountingPoliciesPolicies Significant Accounting Policies (Policies) Policies http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantAccountingPolicies 20 false false R21.htm 100200 - Disclosure - Operating Revenue (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenueTables Operating Revenue (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenue 21 false false R22.htm 100210 - Disclosure - Income Taxes (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureIncomeTaxesTables Income Taxes (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureIncomeTaxes 22 false false R23.htm 100220 - Disclosure - Fair Value Measurements (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsTables Fair Value Measurements (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurements 23 false false R24.htm 100230 - Disclosure - Derivatives and Hedge Accounting Activities (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivitiesTables Derivatives and Hedge Accounting Activities (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivities 24 false false R25.htm 100240 - Disclosure - Regulatory Assets and Liabilities (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryAssetsAndLiabilitiesTables Regulatory Assets and Liabilities (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryAssetsAndLiabilities 25 false false R26.htm 100250 - Disclosure - Significant Financing Transactions (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantFinancingTransactionsTables Significant Financing Transactions (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantFinancingTransactions 26 false false R27.htm 100260 - Disclosure - Related-Party Transactions (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRelatedPartyTransactionsTables Related-Party Transactions (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureRelatedPartyTransactions 27 false false R28.htm 100270 - Disclosure - Operating Segment (Tables) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingSegmentTables Operating Segment (Tables) Tables http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingSegment 28 false false R29.htm 100280 - Disclosure - Operating Revenue - Subsequent to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenueSubsequentToAdoptionOfRevisedGuidanceForRevenueRecognitionFromContractsWithCustomersDetails Operating Revenue - Subsequent to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) Details 29 false false R30.htm 100290 - Disclosure - Operating Revenue - Narrative (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenueNarrativeDetails Operating Revenue - Narrative (Details) Details 30 false false R31.htm 100300 - Disclosure - Operating Revenue - Prior to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingRevenuePriorToAdoptionOfRevisedGuidanceForRevenueRecognitionFromContractsWithCustomersDetails Operating Revenue - Prior to the Adoption of Revised Guidance for Revenue Recognition from Contracts with Customers (Details) Details 31 false false R32.htm 100310 - Disclosure - Income Taxes - Effective Income Tax Rate Reconciliation (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureIncomeTaxesEffectiveIncomeTaxRateReconciliationDetails Income Taxes - Effective Income Tax Rate Reconciliation (Details) Details 32 false false R33.htm 100320 - Disclosure - Income Taxes - Narrative (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureIncomeTaxesNarrativeDetails Income Taxes - Narrative (Details) Details 33 false false R34.htm 100330 - Disclosure - Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsQuantitativeInformationAboutLevel3FairValueMeasurementsDetails Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) Details 34 false false R35.htm 100340 - Disclosure - Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) Details 35 false false R36.htm 100350 - Disclosure - Fair Value Measurements - Net Change in Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsNetChangeInAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisAndIncludedInLevel3FairValueCategoryDetails Fair Value Measurements - Net Change in Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category (Details) Details 36 false false R37.htm 100360 - Disclosure - Fair Value Measurements - Narrative (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsNarrativeDetails Fair Value Measurements - Narrative (Details) Details 37 false false R38.htm 100370 - Disclosure - Fair Value Measurements - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments not Recorded at Fair Value (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfCarryingAmountsAndEstimatedFairValuesOfFinancialInstrumentsNotRecordedAtFairValueDetails Fair Value Measurements - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments not Recorded at Fair Value (Details) Details 38 false false R39.htm 100380 - Disclosure - Derivatives and Hedge Accounting Activities - Schedule of Volume of Derivative Activity (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivitiesScheduleOfVolumeOfDerivativeActivityDetails Derivatives and Hedge Accounting Activities - Schedule of Volume of Derivative Activity (Details) Details 39 false false R40.htm 100390 - Disclosure - Derivatives and Hedge Accounting Activities - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivitiesScheduleOfFairValuesOfDerivativesAndBalanceSheetLocationDetails Derivatives and Hedge Accounting Activities - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) Details 40 false false R41.htm 100400 - Disclosure - Derivatives and Hedge Accounting Activities - Schedule of Gains and Losses on Derivatives (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureDerivativesAndHedgeAccountingActivitiesScheduleOfGainsAndLossesOnDerivativesDetails Derivatives and Hedge Accounting Activities - Schedule of Gains and Losses on Derivatives (Details) Details 41 false false R42.htm 100410 - Disclosure - Regulatory Assets and Liabilities - Regulatory Assets (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryAssetsAndLiabilitiesRegulatoryAssetsDetails Regulatory Assets and Liabilities - Regulatory Assets (Details) Details 42 false false R43.htm 100420 - Disclosure - Regulatory Assets and Liabilities - Regulatory Liabilities (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryAssetsAndLiabilitiesRegulatoryLiabilitiesDetails Regulatory Assets and Liabilities - Regulatory Liabilities (Details) Details 43 false false R44.htm 100430 - Disclosure - Regulatory Matters (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryMattersDetails Regulatory Matters (Details) Details http://www.questargas.com/20180930/taxonomy/role/DisclosureRegulatoryMatters 44 false false R45.htm 100440 - Disclosure - Variable Interest Entities (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureVariableInterestEntitiesDetails Variable Interest Entities (Details) Details http://www.questargas.com/20180930/taxonomy/role/DisclosureVariableInterestEntities 45 false false R46.htm 100450 - Disclosure - Significant Financing Transactions (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantFinancingTransactionsDetails Significant Financing Transactions (Details) Details http://www.questargas.com/20180930/taxonomy/role/DisclosureSignificantFinancingTransactionsTables 46 false false R47.htm 100460 - Disclosure - Commitments and Contingencies (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureCommitmentsAndContingenciesDetails Commitments and Contingencies (Details) Details http://www.questargas.com/20180930/taxonomy/role/DisclosureCommitmentsAndContingencies 47 false false R48.htm 100470 - Disclosure - Related-Party Transactions (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureRelatedPartyTransactionsDetails Related-Party Transactions (Details) Details http://www.questargas.com/20180930/taxonomy/role/DisclosureRelatedPartyTransactionsTables 48 false false R49.htm 100480 - Disclosure - Operating Segment - Narrative (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingSegmentNarrativeDetails Operating Segment - Narrative (Details) Details 49 false false R50.htm 100490 - Disclosure - Operating Segment - Schedule of Segment Information (Details) Sheet http://www.questargas.com/20180930/taxonomy/role/DisclosureOperatingSegmentScheduleOfSegmentInformationDetails Operating Segment - Schedule of Segment Information (Details) Details 50 false false All Reports Book All Reports ck0000068589-20180930.xml ck0000068589-20180930.xsd ck0000068589-20180930_cal.xml ck0000068589-20180930_def.xml ck0000068589-20180930_lab.xml ck0000068589-20180930_pre.xml http://fasb.org/srt/2018-01-31 http://fasb.org/us-gaap/2018-01-31 http://xbrl.sec.gov/dei/2018-01-31 true true