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CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
OPERATING ACTIVITIES      
Net income $ 226.5us-gaap_NetIncomeLoss $ 161.2us-gaap_NetIncomeLoss $ 212.0us-gaap_NetIncomeLoss
Adjustments to reconcile net income to net cash provided by operating activities:      
Depreciation, depletion and amortization 234.4us-gaap_DepreciationDepletionAndAmortization 201.8us-gaap_DepreciationDepletionAndAmortization 189.2us-gaap_DepreciationDepletionAndAmortization
Deferred income taxes 49.2us-gaap_DeferredIncomeTaxExpenseBenefit 42.4us-gaap_DeferredIncomeTaxExpenseBenefit 118.6us-gaap_DeferredIncomeTaxExpenseBenefit
Abandonment and impairment 2.0us-gaap_AssetImpairmentCharges 80.6us-gaap_AssetImpairmentCharges 0us-gaap_AssetImpairmentCharges
Share-based compensation 12.4us-gaap_ShareBasedCompensation 10.2us-gaap_ShareBasedCompensation 9.9us-gaap_ShareBasedCompensation
Net (gain) loss from asset sales (1.2)us-gaap_GainLossOnSaleOfPropertyPlantEquipment 0.2us-gaap_GainLossOnSaleOfPropertyPlantEquipment (5.1)us-gaap_GainLossOnSaleOfPropertyPlantEquipment
(Income) from unconsolidated affiliate (3.5)us-gaap_IncomeLossFromEquityMethodInvestments (3.7)us-gaap_IncomeLossFromEquityMethodInvestments (3.7)us-gaap_IncomeLossFromEquityMethodInvestments
Distributions from unconsolidated affiliate 4.4us-gaap_EquityMethodInvestmentDividendsOrDistributions 4.6us-gaap_EquityMethodInvestmentDividendsOrDistributions 4.5us-gaap_EquityMethodInvestmentDividendsOrDistributions
Other operating 0.9us-gaap_OtherNoncashIncomeExpense 0.9us-gaap_OtherNoncashIncomeExpense 0.4us-gaap_OtherNoncashIncomeExpense
Changes in operating assets and liabilities      
Accounts receivable, including unbilled gas sales 29.5us-gaap_IncreaseDecreaseInAccountsReceivable (41.5)us-gaap_IncreaseDecreaseInAccountsReceivable 23.7us-gaap_IncreaseDecreaseInAccountsReceivable
Inventories (10.4)us-gaap_IncreaseDecreaseInInventories (0.2)us-gaap_IncreaseDecreaseInInventories 2.5us-gaap_IncreaseDecreaseInInventories
Prepaid expenses and other (1.7)us-gaap_IncreaseDecreaseInPrepaidExpense 3.6us-gaap_IncreaseDecreaseInPrepaidExpense (2.8)us-gaap_IncreaseDecreaseInPrepaidExpense
Interest rate hedges - current 0us-gaap_IncreaseDecreaseInRiskManagementAssetsAndLiabilities 0us-gaap_IncreaseDecreaseInRiskManagementAssetsAndLiabilities 7.6us-gaap_IncreaseDecreaseInRiskManagementAssetsAndLiabilities
Accounts payable and accrued expenses (19.6)us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities 33.7us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities (5.1)us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
Federal income taxes (24.0)us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable 6.2us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable (10.7)us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
Current regulatory assets and liabilities (44.5)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities 19.2us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities (24.6)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
Noncurrent regulatory and other assets and liabilities (10.8)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet (17.1)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet (48.7)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet
NET CASH PROVIDED BY OPERATING ACTIVITIES 443.6us-gaap_NetCashProvidedByUsedInOperatingActivities 502.1us-gaap_NetCashProvidedByUsedInOperatingActivities 467.7us-gaap_NetCashProvidedByUsedInOperatingActivities
INVESTING ACTIVITIES      
Property, plant and equipment (319.1)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment (399.4)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment (370.7)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Wexpro acquisition of producing properties (52.4)us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired (104.3)us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired 0us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Cash used in disposition of assets (4.8)us-gaap_PaymentsForProceedsFromRemovalCosts (4.9)us-gaap_PaymentsForProceedsFromRemovalCosts (3.0)us-gaap_PaymentsForProceedsFromRemovalCosts
Proceeds from disposition of assets 9.5us-gaap_ProceedsFromSaleOfProductiveAssets 0.6us-gaap_ProceedsFromSaleOfProductiveAssets 8.4us-gaap_ProceedsFromSaleOfProductiveAssets
NET CASH USED IN INVESTING ACTIVITIES (366.8)us-gaap_NetCashProvidedByUsedInInvestingActivities (508.0)us-gaap_NetCashProvidedByUsedInInvestingActivities (365.3)us-gaap_NetCashProvidedByUsedInInvestingActivities
FINANCING ACTIVITIES      
Common stock issued 2.9us-gaap_ProceedsFromIssuanceOfCommonStock 2.3us-gaap_ProceedsFromIssuanceOfCommonStock 7.2us-gaap_ProceedsFromIssuanceOfCommonStock
Common stock repurchased (4.1)us-gaap_PaymentsForRepurchaseOfCommonStock (4.2)us-gaap_PaymentsForRepurchaseOfCommonStock (87.5)us-gaap_PaymentsForRepurchaseOfCommonStock
Long-term debt issued, net of issuance costs 0us-gaap_ProceedsFromIssuanceOfLongTermDebt 147.3us-gaap_ProceedsFromIssuanceOfLongTermDebt 148.8us-gaap_ProceedsFromIssuanceOfLongTermDebt
Long-term debt repaid 0us-gaap_RepaymentsOfLongTermDebt (42.0)us-gaap_RepaymentsOfLongTermDebt (91.5)us-gaap_RepaymentsOfLongTermDebt
Capital lease obligation repaid (0.9)us-gaap_RepaymentsOfLongTermCapitalLeaseObligations (0.7)us-gaap_RepaymentsOfLongTermCapitalLeaseObligations (0.8)us-gaap_RepaymentsOfLongTermCapitalLeaseObligations
Change in short-term debt 71.0us-gaap_ProceedsFromRepaymentsOfShortTermDebt 13.0us-gaap_ProceedsFromRepaymentsOfShortTermDebt 44.0us-gaap_ProceedsFromRepaymentsOfShortTermDebt
Dividends paid (131.9)us-gaap_PaymentsOfDividends (124.6)us-gaap_PaymentsOfDividends (117.4)us-gaap_PaymentsOfDividends
Tax benefits from share-based compensation 2.2us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities 14.0us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities 0us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES (60.8)us-gaap_NetCashProvidedByUsedInFinancingActivities 5.1us-gaap_NetCashProvidedByUsedInFinancingActivities (97.2)us-gaap_NetCashProvidedByUsedInFinancingActivities
Change in cash and cash equivalents 16.0us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease (0.8)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease 5.2us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
Beginning cash and cash equivalents 16.0us-gaap_CashAndCashEquivalentsAtCarryingValue 16.8us-gaap_CashAndCashEquivalentsAtCarryingValue 11.6us-gaap_CashAndCashEquivalentsAtCarryingValue
Ending cash and cash equivalents 32.0us-gaap_CashAndCashEquivalentsAtCarryingValue 16.0us-gaap_CashAndCashEquivalentsAtCarryingValue 16.8us-gaap_CashAndCashEquivalentsAtCarryingValue
Supplemental Disclosure of Cash Paid (Received) During the Year for:      
Interest 61.3us-gaap_InterestPaidNet 55.8us-gaap_InterestPaidNet 58.5us-gaap_InterestPaidNet
Income taxes 97.4us-gaap_IncomeTaxesPaidNet 33.6us-gaap_IncomeTaxesPaidNet 9.1us-gaap_IncomeTaxesPaidNet
Supplemental Disclosure of Noncash Investing and Financing Transaction:      
Capital lease obligation incurred     40.8us-gaap_CapitalLeaseObligationsIncurred
Questar Gas [Member]      
OPERATING ACTIVITIES      
Net income 55.2us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarGasMember
52.8us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarGasMember
47.1us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarGasMember
Adjustments to reconcile net income to net cash provided by operating activities:      
Depreciation and amortization 58.8str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
54.7str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
52.1str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
Deferred income taxes 46.0us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarGasMember
37.7us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarGasMember
45.9us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarGasMember
Share-based compensation 1.6us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarGasMember
1.4us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarGasMember
1.2us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarGasMember
Net (gain) loss from asset sales (0.1)us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
0us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
0us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
Changes in operating assets and liabilities      
Accounts receivable, including unbilled gas sales 4.6us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarGasMember
(43.7)us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarGasMember
(12.7)us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarGasMember
Inventories (8.2)us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarGasMember
0.5us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarGasMember
0.7us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarGasMember
Prepaid expenses and other (0.5)us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
0.2us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
(0.1)us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarGasMember
Accounts payable and accrued expenses (0.2)us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
29.5us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
7.8us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
Federal income taxes 9.2us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarGasMember
(2.8)us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarGasMember
1.5us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarGasMember
Current regulatory assets and liabilities (46.7)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
17.6us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
(24.4)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarGasMember
Noncurrent regulatory and other assets and liabilities (6.0)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
5.4us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
(6.4)us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
NET CASH PROVIDED BY OPERATING ACTIVITIES 113.7us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
153.3us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
112.7us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
INVESTING ACTIVITIES      
Property, plant and equipment (174.7)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
(166.2)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
(162.1)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarGasMember
Cash used in disposition of assets (3.4)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarGasMember
(3.9)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarGasMember
(2.3)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarGasMember
Proceeds from disposition of assets 0.8us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarGasMember
0.3us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarGasMember
0.4us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarGasMember
Affiliated-company property, plant and equipment transfers 0str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarGasMember
10.8str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarGasMember
0str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarGasMember
NET CASH USED IN INVESTING ACTIVITIES (177.3)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
(159.0)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
(164.0)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
FINANCING ACTIVITIES      
Long-term debt issued, net of issuance costs 0us-gaap_ProceedsFromIssuanceOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
149.0us-gaap_ProceedsFromIssuanceOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
148.8us-gaap_ProceedsFromIssuanceOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
Long-term debt repaid 0us-gaap_RepaymentsOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
(42.0)us-gaap_RepaymentsOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
(91.5)us-gaap_RepaymentsOfLongTermDebt
/ dei_LegalEntityAxis
= str_QuestarGasMember
Change in notes payable to Questar 101.6us-gaap_IncreaseDecreaseInNotesPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarGasMember
(58.4)us-gaap_IncreaseDecreaseInNotesPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarGasMember
23.4us-gaap_IncreaseDecreaseInNotesPayableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarGasMember
Dividends paid (27.0)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarGasMember
(35.5)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarGasMember
(33.0)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarGasMember
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES 74.6us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
13.1us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
47.7us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarGasMember
Change in cash and cash equivalents 11.0us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarGasMember
7.4us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarGasMember
(3.6)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarGasMember
Beginning cash and cash equivalents 8.8us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
1.4us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
5.0us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
Ending cash and cash equivalents 19.8us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
8.8us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
1.4us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarGasMember
Supplemental Disclosure of Cash Paid (Received) During the Year for:      
Interest 26.3us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
21.0us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
24.7us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
Income taxes (13.2)us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
(13.8)us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
(20.4)us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarGasMember
Supplemental Disclosure of Noncash Investing and Financing Transaction:      
Noncash reduction in notes payable to Questar due to equity contribution   90.0us-gaap_NotesReduction
/ dei_LegalEntityAxis
= str_QuestarGasMember
 
Questar Pipeline [Member]      
OPERATING ACTIVITIES      
Net income 60.6us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
8.2us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
64.7us-gaap_NetIncomeLoss
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Adjustments to reconcile net income to net cash provided by operating activities:      
Depreciation and amortization 56.9str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
58.1str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
56.8str_DepreciationAndAmortizationExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Deferred income taxes 12.6us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(14.3)us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
41.1us-gaap_DeferredIncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Abandonment and impairment 0us-gaap_AssetImpairmentCharges
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
80.6us-gaap_AssetImpairmentCharges
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0us-gaap_AssetImpairmentCharges
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Share-based compensation 2.1us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
2.0us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
2.0us-gaap_ShareBasedCompensation
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Net (gain) loss from asset sales 0.5us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(2.7)us-gaap_GainLossOnSaleOfPropertyPlantEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(Income) from unconsolidated affiliate (3.5)us-gaap_IncomeLossFromEquityMethodInvestments
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(3.7)us-gaap_IncomeLossFromEquityMethodInvestments
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(3.7)us-gaap_IncomeLossFromEquityMethodInvestments
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Distributions from unconsolidated affiliate 4.4us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
4.6us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
4.5us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Other operating 0.5us-gaap_OtherNoncashIncomeExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0.5us-gaap_OtherNoncashIncomeExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0.4us-gaap_OtherNoncashIncomeExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Changes in operating assets and liabilities      
Accounts receivable, including unbilled gas sales (1.3)us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(8.5)us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(2.1)us-gaap_IncreaseDecreaseInAccountsReceivable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Inventories (2.2)us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(0.9)us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0.2us-gaap_IncreaseDecreaseInInventories
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Prepaid expenses and other 0.1us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
3.3us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(3.3)us-gaap_IncreaseDecreaseInPrepaidExpense
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Accounts payable and accrued expenses (7.6)us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0.4us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(3.4)us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Federal income taxes (3.6)us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
2.9us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Current regulatory assets and liabilities 4.5us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
2.1us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(0.9)us-gaap_IncreaseDecreaseInRegulatoryAssetsAndLiabilities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
NET CASH PROVIDED BY OPERATING ACTIVITIES 124.0us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
135.3us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
153.6us-gaap_NetCashProvidedByUsedInOperatingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
INVESTING ACTIVITIES      
Property, plant and equipment (59.4)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(73.4)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(60.6)us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Cash used in disposition of assets (1.4)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(1.0)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(0.7)us-gaap_PaymentsForProceedsFromRemovalCosts
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Proceeds from disposition of assets 0.2us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0.1us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
4.3us-gaap_ProceedsFromSaleOfProductiveAssets
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Affiliated-company property, plant and equipment transfers 0str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(10.7)str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
0str_ProceedsFromPaymentsForProductiveAssetsAffiliatedCompanies
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
NET CASH USED IN INVESTING ACTIVITIES (60.6)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(85.0)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(57.0)us-gaap_NetCashProvidedByUsedInInvestingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
FINANCING ACTIVITIES      
Change in notes receivable from Questar (10.7)us-gaap_IncreaseDecreaseInNotesReceivableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
9.3us-gaap_IncreaseDecreaseInNotesReceivableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(29.9)us-gaap_IncreaseDecreaseInNotesReceivableRelatedPartiesCurrent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Dividends paid (48.0)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(64.0)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(63.0)us-gaap_PaymentsOfDividends
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
NET CASH PROVIDED BY (USED IN) FINANCING ACTIVITIES (58.7)us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(54.7)us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(92.9)us-gaap_NetCashProvidedByUsedInFinancingActivities
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Change in cash and cash equivalents 4.7us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
(4.4)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
3.7us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Beginning cash and cash equivalents 2.7us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
7.1us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
3.4us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Ending cash and cash equivalents 7.4us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
2.7us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
7.1us-gaap_CashAndCashEquivalentsAtCarryingValue
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Supplemental Disclosure of Cash Paid (Received) During the Year for:      
Interest 25.4us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
25.2us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
25.2us-gaap_InterestPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Income taxes $ 32.4us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
$ 11.0us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
$ (3.8)us-gaap_IncomeTaxesPaidNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember