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Debt Long-term Debt (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Debt Instrument [Line Items]    
Total long-term debt outstanding $ 1,136.6 $ 1,078.1
Less current portion (42.0) (91.5)
Less unamortized-debt discount (2.3) (2.4)
Plus unamortized-debt premium 0.8 1.0
Plus fair value hedge adjustment 5.8 7.8
Total long-term debt, less current portion 1,098.9 993.0
Questar Corp [Member] | 2.75% Notes due 2016 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 2.75%  
Debt Instrument, Maturity Date Feb. 01, 2016  
Total long-term debt outstanding 250.0 250.0
Questar Gas [Member]
   
Debt Instrument [Line Items]    
Total long-term debt outstanding 426.5 368.0
Less current portion (42.0) (91.5)
Total long-term debt, less current portion 384.5 276.5
Questar Gas [Member] | 6.30% to 6.91% Medium-term Notes due 2012 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum 6.30%  
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum 6.91%  
Debt Instrument, Maturity Date Oct. 01, 2012  
Long-Term Debt Amounts Repaid During the Year 91.5  
Total long-term debt outstanding 0 91.5
Questar Gas [Member] | 5.00% to 6.89% Medium-term Notes due 2013 to 2018 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum 5.00%  
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum 6.89%  
Debt Instrument, Maturity Date Range, Start Jan. 24, 2013  
Debt Instrument, Maturity Date Range, End Mar. 15, 2018  
Total long-term debt outstanding 126.5 126.5
Questar Gas [Member] | 6.30% notes due 2018 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 6.30%  
Debt Instrument, Maturity Date Apr. 01, 2018  
Total long-term debt outstanding 50.0 50.0
Questar Gas [Member] | 2.98% Notes due 2024 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 2.98%  
Debt Instrument, Maturity Date Dec. 01, 2024  
Debt Instrument, Face Amount 40.0  
Debt Instrument, Term 12 years  
Total long-term debt outstanding 40.0 0
Questar Gas [Member] | 3.28% Notes due 2027 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.28%  
Debt Instrument, Maturity Date Dec. 01, 2027  
Debt Instrument, Face Amount 110.0  
Debt Instrument, Term 15 years  
Total long-term debt outstanding 110.0 0
Questar Gas [Member] | 7.20% notes due 2038 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 7.20%  
Debt Instrument, Maturity Date Apr. 01, 2038  
Total long-term debt outstanding 100.0 100.0
Questar Gas [Member] | Questar Gas Notes Due January 2013 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Maturity Date Jan. 24, 2013  
Long-Term Debt Amounts Repaid During the Year 40.0  
Questar Gas [Member] | Questar Gas Notes due September 2013 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Maturity Date Sep. 30, 2013  
Long-Term Debt Amounts Repaid During the Year 2.0  
Questar Gas [Member] | Questar Gas Notes refinanced in December 2012 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 6.06%  
Questar Gas [Member] | 2.98% and 3.28% Notes due 2024 and 2027 [Member]
   
Debt Instrument [Line Items]    
Net Proceeds from Issuance of Long-Term Debt 148.8  
Questar Pipeline [Member]
   
Debt Instrument [Line Items]    
Total long-term debt outstanding 460.1 460.1
Total long-term debt, less current portion 458.9 459.0
Questar Pipeline [Member] | 6.45% and 6.48% Medium-term Notes due 2015 and 2018 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum 6.45%  
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum 6.48%  
Debt Instrument, Maturity Date Range, Start Oct. 15, 2015  
Debt Instrument, Maturity Date Range, End Dec. 11, 2018  
Total long-term debt outstanding 30.1 30.1
Questar Pipeline [Member] | 5.83% Notes due 2018 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 5.83%  
Debt Instrument, Maturity Date Feb. 01, 2018  
Total long-term debt outstanding 250.0 250.0
Questar Pipeline [Member] | 4.875% Notes due 2041 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 4.875%  
Debt Instrument, Maturity Date Dec. 01, 2041 Dec. 01, 2041
Debt Instrument, Face Amount   180.0
Debt Instrument, Interest Rate, Effective Percentage   6.66%
Net Proceeds from Issuance of Long-Term Debt   138.9
Total long-term debt outstanding 180.0 180.0
Questar Pipeline [Member] | Questar Pipeline 7.09% Medium-Term Notes due 2011 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage   7.09%
Debt Instrument, Maturity Date   Jun. 01, 2011
Long-Term Debt Amounts Repaid During the Year   100.0
Questar Pipeline [Member] | Questar Pipeline 6.57% Medium-Term Notes due 2011 [Member]
   
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage   6.57%
Debt Instrument, Maturity Date   Sep. 26, 2011
Long-Term Debt Amounts Repaid During the Year   $ 80.0