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Share-Based Compensation Performance shares (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 31, 2015
Performance Shares [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share) $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Risk-free interest rate (in hundredths) 0.67%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
0.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
0.35%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Expected price volatility (in hundredths) 18.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
19.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
22.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Expected dividend yield (in hundredths) 3.05%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
2.88%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
3.35%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Expected life (in years and months) 2 years 11 months 2 years 11 months 2 years 11 months  
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2013 387,879us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Granted 139,611us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Payout adjustment (76,657)str_SharebasedCompensationArrangmentbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsPayoutAdjustmentinPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]      
Distributed (51,104)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]      
Forfeited (8,480)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Balance at December 31, 2014 391,249us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[2],[3] 387,879us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
   
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2013 $ 27.61us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Granted $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Payout adjustment $ 18.23str_SharebasedCompensationArrangementbySharebasedAwardEquityInstrumentsOtherthanOptionsPayoutAdjustmentinPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]      
Distributed $ 18.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]      
Forfeited $ 33.33us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Balance at December 31, 2014 $ 31.78us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[2],[3] $ 27.61us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
   
Performance shares, additional disclosures [Abstract]        
Performance share payout adjustment multiplier 0.40str_PerformanceSharePayoutAdjustmentFactor
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Performance period that the target number of performance shares for each exectutive officer is subject to adjustment based on the company's performance (in years) 3 years      
Equity- and liability-based performance share compensation expense $ 3.8us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 2.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
$ 1.8us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
 
Weighted-average remaining vesting period of share-based awards issued (in months) 18 months      
Performance share cash distributed $ 0.6us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
     
Performance Shares [Member] | Vested Undistributed [Member]        
Performance shares, additional disclosures [Abstract]        
Performance share payout adjustment multiplier       1.00str_PerformanceSharePayoutAdjustmentFactor
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= str_VestedUndistributedMember
Number of undistributed performance shares at end of period (in shares) 134,778us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= str_VestedUndistributedMember
     
The date the share-based award will be distributed       first quarter of 2015
Performance Shares [Member] | Minimum [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share) $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2013 $ 18.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Granted $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Payout adjustment $ 18.23str_SharebasedCompensationArrangementbySharebasedAwardEquityInstrumentsOtherthanOptionsPayoutAdjustmentinPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[1]      
Distributed $ 18.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[1]      
Forfeited $ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Balance at December 31, 2014 $ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
[2],[3]      
Performance shares, additional disclosures [Abstract]        
Performance share payout adjustment multiplier 0.00str_PerformanceSharePayoutAdjustmentFactor
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Performance Shares [Member] | Maximum [Member]        
Major assumptions used in the calculation of fair value of performance shares granted [Abstract]        
Weighted-average grant date fair value, performance shares granted (in dollars per share) $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2013 $ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Granted $ 31.07us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Payout adjustment $ 18.23str_SharebasedCompensationArrangementbySharebasedAwardEquityInstrumentsOtherthanOptionsPayoutAdjustmentinPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[1]      
Distributed $ 18.23us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[1]      
Forfeited $ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Balance at December 31, 2014 $ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[2],[3]      
Performance shares, additional disclosures [Abstract]        
Performance share payout adjustment multiplier 3.00str_PerformanceSharePayoutAdjustmentFactor
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Performance Shares - Cash Settled [Member]        
Performance shares, additional disclosures [Abstract]        
Number of undistributed performance shares at end of period (in shares) 128,270us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= str_PerformanceSharesCashSettledMember
     
Performance Shares - Equity Settled [Member]        
Performance shares, additional disclosures [Abstract]        
Number of undistributed performance shares at end of period (in shares) 262,979us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= str_PerformanceSharesEquitySettledMember
     
Questar Gas [Member] | Performance Shares [Member]        
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2014 33,826us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarGasMember
     
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 31.73us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarGasMember
     
Performance shares, additional disclosures [Abstract]        
Weighted-average remaining vesting period of share-based awards issued (in months) 18 months      
Questar Gas [Member] | Performance Shares [Member] | Minimum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarGasMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Questar Gas [Member] | Performance Shares [Member] | Maximum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarGasMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
Questar Pipeline [Member] | Performance Shares [Member]        
Transactions under the terms of the LTSIP - shares [Roll Forward]        
Balance at December 31, 2014 59,956us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
     
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 31.59us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
     
Performance shares, additional disclosures [Abstract]        
Weighted-average remaining vesting period of share-based awards issued (in months) 18 months      
Questar Pipeline [Member] | Performance Shares [Member] | Minimum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 25.42us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
     
Questar Pipeline [Member] | Performance Shares [Member] | Maximum [Member]        
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]        
Balance at December 31, 2014 $ 39.62us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ dei_LegalEntityAxis
= str_QuestarPipelineMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
     
[1] Actual shares and cash distributed were determined by multiplying the target shares by 0.40 to reflect Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2011 through December 31, 2013. The total amount paid for the cash half of the distribution was $0.6 million.
[2] The total undistributed balance consists of 262,979 of share-settled target shares and 128,270 of cash-settled target shares.
[3] The total undistributed balance includes 134,778 vested target shares that are expected to be distributed in the first quarter of 2015 using a payout adjustment multiplier of 1.00. The multiplier reflects Questar's final total shareholder return ranking relative to the specified peer companies for the performance period from January 1, 2012 through December 31, 2014. The distribution will be half in shares, half in cash. These target shares are included in the amounts described in note (2), immediately above.