Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of Income Tax Expense [Abstract]
Current	$ (22.3)	$ 18.3	$ 19.7
Deferred	133.1	85.6	77.5
State [Abstract]
Current	(1.3)	(0.2)	2.6
Deferred	7.3	6.1	5.0
Deferred investment tax credits recognized	(0.4)	(0.4)	(0.4)
Total income tax expense	116.4	109.4	104.4
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.30%	1.30%	1.70%
Tax benefits from dividends paid to employee stock plan (in hundredths)	(0.50%)	(0.40%)	0.00%
Other (in hundredths)	0.10%	0.40%	(0.10%)
Effective income tax rate (in hundredths)	35.90%	36.30%	36.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	673.6	534.5
Deferred tax liabilities - noncurrent	673.6	534.5
Deferred tax assets [Abstract]
Employee benefits and compensation costs	116.1	59.8
Hedging activities	13.8	0
Net operating loss carryforward	39.2	0
Other	4.3	0
Deferred tax assets - noncurrent	173.4	59.8
Deferred income taxes - noncurrent	500.2	474.7
Deferred income taxes - current asset	16.1	14.8
Deferred income taxes - current liability	0	3.0
Deferred income taxes - current	16.1	11.8
Questar Pipeline [Member]
Components of Income Tax Expense [Abstract]
Current	(25.9)	19.7	6.1
Deferred	62.9	16.7	25.3
State [Abstract]
Current	(0.7)	1.5	1.3
Deferred	1.9	0.9	1.1
Total income tax expense	38.2	38.8	33.8
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.00%	1.50%	1.70%
Effective income tax rate (in hundredths)	36.00%	36.50%	36.70%
Deferred tax liabilities [Abstract]
Property, plant and equipment	240.4	178.1
Employee benefits and compensation costs	3.8	2.0
Deferred tax liabilities - noncurrent	244.2	180.1
Deferred tax assets [Abstract]
Hedging activities	13.8	0
Net operating loss carryforward	29.3	0
Other	(0.3)	0
Deferred tax assets - noncurrent	42.8	0
Deferred income taxes - noncurrent	201.4	180.1
Deferred income taxes - current	1.8	1.2
Questar Gas [Member]
Components of Income Tax Expense [Abstract]
Current	(10.5)	(15.9)	(8.3)
Deferred	36.8	40.2	31.6
State [Abstract]
Current	(1.6)	(2.2)	(1.2)
Deferred	3.2	3.5	2.7
Deferred investment tax credits recognized	(0.4)	(0.4)	(0.4)
Total income tax expense	27.5	25.2	24.4
Difference between the statutory federal income tax rate and the Company's effective income tax rate [Abstract]
Federal income taxes statutory rate (in hundredths)	35.00%	35.00%	35.00%
State income taxes, net of federal income tax benefit (in hundredths)	1.40%	1.20%	1.50%
Amortize investment tax credits related to rate regulated assets (in hundredths)	(0.50%)	(0.50%)	(0.60%)
Other (in hundredths)	1.50%	0.80%	1.10%
Effective income tax rate (in hundredths)	37.40%	36.50%	37.00%
Deferred tax liabilities [Abstract]
Property, plant and equipment	264.7	225.0
Employee benefits and compensation costs	9.7	5.3
Deferred tax liabilities - noncurrent	274.4	230.3
Deferred tax assets [Abstract]
Net operating loss carryforward	14.6	0
Deferred tax assets - noncurrent	14.6	0
Deferred income taxes - noncurrent	259.8	230.3
Deferred income taxes - current asset	5.6	4.3
Deferred income taxes - current liability	0	3.0
Deferred income taxes - current	$ 5.6	$ 1.3