Debt (Details) (USD $) In Millions, unless otherwise specified			12 Months Ended				12 Months Ended								12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended						12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Revolving credit arrangement [Member]	Dec. 31, 2010 Revolving credit arrangement [Member]	Dec. 31, 2011 Commercial paper [Member]	Dec. 31, 2010 Commercial paper [Member]	Dec. 31, 2011 2.75% Notes due 2016 [Member]	Dec. 31, 2011 Questar Corp [Member] 2.75% Notes due 2016 [Member]	Dec. 31, 2010 Questar Corp [Member] 2.75% Notes due 2016 [Member]	Dec. 31, 2011 Questar Pipeline [Member]	Dec. 31, 2010 Questar Pipeline [Member]	Dec. 31, 2011 Questar Pipeline [Member] Notes payable to Questar [Member]	Nov. 30, 2011 Questar Pipeline [Member] Notes payable to Questar [Member]	Dec. 31, 2010 Questar Pipeline [Member] Notes payable to Questar [Member]	Dec. 31, 2011 Questar Pipeline [Member] 6.45% and 6.48% Medium-term Notes due 2015 and 2018 [Member]	Dec. 31, 2010 Questar Pipeline [Member] 6.45% and 6.48% Medium-term Notes due 2015 and 2018 [Member]	Dec. 31, 2011 Questar Pipeline [Member] 6.57% and 7.09% Medium-term Notes due 2011 [Member]	Dec. 31, 2010 Questar Pipeline [Member] 6.57% and 7.09% Medium-term Notes due 2011 [Member]	Dec. 31, 2011 Questar Pipeline [Member] 5.83% Notes due 2018 [Member]	Dec. 31, 2010 Questar Pipeline [Member] 5.83% Notes due 2018 [Member]	Dec. 31, 2011 Questar Pipeline [Member] 4.875% Notes due 2041 [Member]	Dec. 31, 2010 Questar Pipeline [Member] 4.875% Notes due 2041 [Member]	Dec. 31, 2011 Questar Pipeline [Member] Questar Pipeline 7.09% Medium-Term Notes Due 2011 [Member]	Dec. 31, 2011 Questar Pipeline [Member] Questar Pipeline 6.57% Medium-Term Notes Due 2011 [Member]	Dec. 31, 2011 Questar Gas [Member]	Dec. 31, 2010 Questar Gas [Member]	Dec. 31, 2011 Questar Gas [Member] Notes payable to Questar [Member]	Dec. 31, 2010 Questar Gas [Member] Notes payable to Questar [Member]	Dec. 31, 2011 Questar Gas [Member] 5.00% to 6.91% Medium-term Notes due 2012 to 2018 [Member]	Dec. 31, 2010 Questar Gas [Member] 5.00% to 6.91% Medium-term Notes due 2012 to 2018 [Member]	Dec. 31, 2011 Questar Gas [Member] 6.89% Medium-term Notes due 2011 [Member]	Dec. 31, 2010 Questar Gas [Member] 6.89% Medium-term Notes due 2011 [Member]	Dec. 31, 2011 Questar Gas [Member] 6.30% notes due 2018 [Member]	Dec. 31, 2010 Questar Gas [Member] 6.30% notes due 2018 [Member]	Dec. 31, 2011 Questar Gas [Member] 7.20% notes due 2038 [Member]	Dec. 31, 2010 Questar Gas [Member] 7.20% notes due 2038 [Member]
Debt [Line Items]
Revolving credit arrangement credit commitments			$ 500.0	$ 350.0
Covenant compliance, revolving credit arrangement			Consolidated funded debt cannot exceed 70% of consolidated capitalization. Questar is in compliance with this covenant at December 31, 2011.
Commercial paper with variable-interest rates	219.0	242.0			219.0	242.0
Notes payable to parent												0		0											142.7	153.6	142.7	153.6
Weighted-average interest rate					0.29%	0.38%							0.35%														0.41%	0.48%
Debt Instrument, Interest Rate, Stated Percentage							2.75%												5.83%		4.875%										6.89%		6.30%		7.20%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum															0.0645		0.0657												0.0500
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum															0.0648		0.0709												0.0691
Debt Instrument Maturity Dates			2016-08	2013-07			2016												2018		2041-12										2011		2018		2038
Debt Instrument, Maturity Date Range, Start															2015		2011-06												2012
Debt Instrument, Maturity Date Range, End															2018		2011-09												2018
Debt Instrument, Face Amount																					180.0
Debt Instrument, Interest Rate, Effective Percentage																					6.66%
Net Proceeds from Public Issuance of Long-Term Debt																					138.9
Intercompany short-term debt repaid with proceeds													145.5
Long-Term Debt Amounts Repaid During the Year																							100.0	80.0
Total long-term debt outstanding	1,078.1	1,080.1						250.0	250.0	460.1	460.1				30.1	30.1	0	180.0	250.0	250.0	180.0	0			368.0	370.0			218.0	218.0	0	2.0	50.0	50.0	100.0	100.0
Less current portion	(91.5)	(182.0)								0	(180.0)														(91.5)	(2.0)
Less unamortized-debt discount	(2.4)	(0.7)
Plus unamortized-debt premium	1.0	1.1
Plus fair value hedge adjustment	7.8	0
Total long-term debt	$ 993.0	$ 898.5								$ 459.0	$ 280.9														$ 276.5	$ 368.0