XML 35 R48.htm IDEA: XBRL DOCUMENT v2.4.1.9
Accumulated Other Comprehensive Income (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances $ (141.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax $ (246.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax  
OCI before reclassifications (113.6)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax 135.7us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax  
Reclassified from AOCI 17.2us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax [1] 33.5us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax [1]  
Income taxes      
Attributable to OCI before reclassifications 43.4str_OtherComprehensiveIncomeLossBeforeReclassificationsTax (51.8)str_OtherComprehensiveIncomeLossBeforeReclassificationsTax  
Attributable to amounts reclassified from AOCI (6.5)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax [2] (12.9)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax [2]  
Total income taxes 36.9us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 (64.7)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 12.3us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
Net other comprehensive income (loss) (59.5)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent 104.5us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent (20.2)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
Ending balances (201.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (141.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (246.3)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]      
Income tax (expense) benefit (125.9)us-gaap_IncomeTaxExpenseBenefit (101.3)us-gaap_IncomeTaxExpenseBenefit (116.5)us-gaap_IncomeTaxExpenseBenefit
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Change in unrecognized actuarial loss     13.6us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodTax
Change in unrecognized prior service cost     (1.0)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansBenefitPlanImprovementTaxEffect
Interest rate cash flow hedges     (0.2)us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesTax
Change in fair value of long-term investment     (0.1)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Total income taxes 36.9us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 (64.7)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1 12.3us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
Pension Plans, Defined Benefit [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (108.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 (203.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
OCI before reclassifications (111.5)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 123.4us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Reclassified from AOCI 16.0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [1] 30.0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 42.6str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 (47.2)str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Attributable to amounts reclassified from AOCI (6.1)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [2] (11.5)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [2]  
Total income taxes 36.5us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 (58.7)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Net other comprehensive income (loss) (59.0)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 94.7us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Ending balances (167.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 (108.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes 36.5us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 (58.7)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Actuarial Loss [Member] | Pension Plans, Defined Benefit [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (108.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (202.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
OCI before reclassifications (111.5)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 123.4us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Reclassified from AOCI 15.4us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [1] 28.9us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 42.6str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (47.2)str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Attributable to amounts reclassified from AOCI (5.9)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [2] (11.1)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [2]  
Total income taxes 36.7us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (58.3)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Net other comprehensive income (loss) (59.4)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 94.0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Ending balances (167.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (108.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes 36.7us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (58.3)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Actuarial Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (10.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (19.7)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
OCI before reclassifications (2.1)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 12.4us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Reclassified from AOCI 0.7us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [1] 3.0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 0.8str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (4.7)str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Attributable to amounts reclassified from AOCI (0.3)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [2] (1.2)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 [2]  
Total income taxes 0.5us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (5.9)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Net other comprehensive income (loss) (0.9)us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 9.5us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Ending balances (11.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (10.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes 0.5us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		 (5.9)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetUnamortizedGainLossMember		  
Prior service cost [Member] | Pension Plans, Defined Benefit [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (0.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 (1.1)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
OCI before reclassifications 0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Reclassified from AOCI 0.6us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 [1] 1.1us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 0str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 0str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Attributable to amounts reclassified from AOCI (0.2)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 [2] (0.4)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 [2]  
Total income taxes (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 (0.4)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Net other comprehensive income (loss) 0.4us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 0.7us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Ending balances 0us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 (0.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		 (0.4)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentNetPriorServiceCostCreditMember		  
Long-term investment [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances 0.1us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0.1us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
OCI before reclassifications 0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 (0.1)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Reclassified from AOCI 0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 [1] 0us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 0str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0.1str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Attributable to amounts reclassified from AOCI 0str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 [2] 0str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 [2]  
Total income taxes 0us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0.1us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Net other comprehensive income (loss) 0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Ending balances 0.1us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0.1us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes 0us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		 0.1us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember		  
Questar Pipeline [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (22.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		    
Income taxes      
Total income taxes (0.1)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Net other comprehensive income (loss) 0.4us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 0.3us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 0.2us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Ending balances (22.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (22.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		  
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]      
Income tax (expense) benefit (35.6)us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (7.0)us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (37.3)us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes (0.1)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
Questar Pipeline [Member] | Interest rate cash flow hedges [Member]      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]      
Beginning balances (22.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (23.2)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
OCI before reclassifications 0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Reclassified from AOCI 0.5us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1] 0.5us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [1]  
Income taxes      
Attributable to OCI before reclassifications 0str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 0str_OtherComprehensiveIncomeLossBeforeReclassificationsTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Attributable to amounts reclassified from AOCI (0.1)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [2] (0.2)str_ReclassificationsFromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 [2]  
Total income taxes (0.1)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Net other comprehensive income (loss) 0.4us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 0.3us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Ending balances (22.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (22.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]      
Total income taxes (0.1)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (0.2)us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Questar Pipeline [Member] | Interest rate cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]      
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]      
Interest income (expense) (0.5)us-gaap_InterestIncomeExpenseNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 (0.5)us-gaap_InterestIncomeExpenseNet
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
Income tax (expense) benefit $ 0.1us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		 $ 0.2us-gaap_IncomeTaxExpenseBenefit
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis
= us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember		  
[1] Interest rate cash flow hedge amounts are included in their entirety as charges to interest expense on the Consolidated Statements of Income.
[2] Income tax reclassifications related to interest rate cash flow hedge amounts are included in their entirety as credits to income taxes on the Consolidated Statements of Income.