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Employee Benefits Defined benefit plans disclosures (Details)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Total debt securities [Abstract]    
Time over which set performance objectives for each investment manager are expected to be met (in years) 3 years  
Minimum [Member]    
Total debt securities [Abstract]    
Average maturities for investment-grade intermediate-term debt (in years) 5 years  
Average maturities for investment-grade long-term debt (in years) 10 years  
Maximum [Member]    
Total debt securities [Abstract]    
Average maturities for investment-grade intermediate-term debt (in years) 10 years  
Average maturities for below-investment-grade debt (in years) 10 years  
Total domestic equity securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 38.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		 38.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
Policy Range [Abstract]    
Total domestic equity securities - minimum range (in hundredths) 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
Total domestic equity securities - maximum range (in hundredths) 45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember		 45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
Total foreign securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 22.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember		 25.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember
Policy Range [Abstract]    
Total domestic equity securities - minimum range (in hundredths) 25.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember		 25.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember
Total domestic equity securities - maximum range (in hundredths) 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember		 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_ForeignSecuritiesMember
Developed market foreign equity securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 18.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_DevelopedMarketForeignEquitySecuritiesMember		 20.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_DevelopedMarketForeignEquitySecuritiesMember
Emerging market foreign equity securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 4.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_EmergingMarketForeignEquitySecuritiesMember		 5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_EmergingMarketForeignEquitySecuritiesMember
Total debt securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 30.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember		 27.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember
Policy Range [Abstract]    
Total domestic equity securities - minimum range (in hundredths) 25.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember		 25.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember
Total domestic equity securities - maximum range (in hundredths) 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember		 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_DebtSecuritiesMember
Investment grade intermediate term debt [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 7.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InvestmentGradeIntermediateTermDebtMember		 7.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InvestmentGradeIntermediateTermDebtMember
Investment grade long-term debt [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 14.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InvestmentGradeLongTermDebtMember		 11.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InvestmentGradeLongTermDebtMember
Below-Investment grade debt [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_BelowInvestmentGradeDebtMember		 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_BelowInvestmentGradeDebtMember
Inflation protection securities [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember		 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember
Policy Range [Abstract]    
Total domestic equity securities - minimum range (in hundredths) 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember		 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember
Total domestic equity securities - maximum range (in hundredths) 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember		 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_InflationProtectionFundsMember
Cash and short-term investments [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Total domestic equity securities (in hundredths) 1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember		 1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember
Policy Range [Abstract]    
Total domestic equity securities - minimum range (in hundredths) 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember		 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember
Total domestic equity securities - maximum range (in hundredths) 3.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember		 3.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= str_CashAndShortTermInvestmentsMember