Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Federal [Abstract]
Current	$ 1.3	$ 0	$ 18.3
Deferred	112.0	110.8	85.6
State [Abstract]
Current	0	(1.3)	(0.2)
Deferred	3.6	7.3	6.1
Deferred investment tax credits recognized	(0.4)	(0.4)	(0.4)
Total income tax expense	116.5	116.4	109.4
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)	1.00%	1.30%	1.30%
Tax benefits from dividends paid to employee stock plan (in hundredths)	(0.50%)	(0.50%)	(0.40%)
Other (in hundredths)	0.00%	0.10%	0.40%
Effective income tax rate (in hundredths)	35.50%	35.90%	36.30%
Deferred tax liabilities [Abstract]
Property, plant and equipment	743.1	673.6
Deferred tax liabilities - noncurrent	743.1	673.6
Deferred tax assets [Abstract]
Employee benefits and compensation costs	110.0	116.1
Hedging activities	13.6	13.8
Net operating loss carryforward	15.8	39.2
Other	0.3	4.3
Deferred tax assets - noncurrent	139.7	173.4
Deferred income taxes - net noncurrent liability	603.4	500.2
Deferred income taxes - current asset	17.6	16.1
Deferred income taxes - current liability	4.6	0
Deferred income taxes - net current asset	13.0	16.1
Questar Gas [Member]
Federal [Abstract]
Current	(18.7)	0	(15.9)
Deferred	45.1	26.3	40.2
State [Abstract]
Current	0	(1.6)	(2.2)
Deferred	1.2	3.2	3.5
Deferred investment tax credits recognized	(0.4)	(0.4)	(0.4)
Total income tax expense	27.2	27.5	25.2
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%	1.40%	1.20%
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	(0.40%)	(0.50%)	(0.50%)
Other (in hundredths)	1.00%	1.50%	0.80%
Effective income tax rate (in hundredths)	36.60%	37.40%	36.50%
Deferred tax liabilities [Abstract]
Property, plant and equipment	293.4	264.7
Employee benefits and compensation costs	19.3	9.7
Deferred tax liabilities - noncurrent	312.7	274.4
Deferred tax assets [Abstract]
Net operating loss carryforward	11.1	14.6
Deferred tax assets - noncurrent	11.1	14.6
Deferred income taxes - net noncurrent liability	301.6	259.8
Deferred income taxes - current asset	6.1	5.6
Deferred income taxes - current liability	4.6	0
Deferred income taxes - net current asset	1.5	5.6
Questar Pipeline [Member]
Federal [Abstract]
Current	(5.4)	0	19.7
Deferred	40.4	37.0	16.7
State [Abstract]
Current	1.6	(0.7)	1.5
Deferred	0.7	1.9	0.9
Total income tax expense	37.3	38.2	38.8
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)	1.60%	1.00%	1.50%
Effective income tax rate (in hundredths)	36.60%	36.00%	36.50%
Deferred tax liabilities [Abstract]
Property, plant and equipment	250.2	240.4
Employee benefits and compensation costs	6.8	3.8
Deferred tax liabilities - noncurrent	257.0	244.2
Deferred tax assets [Abstract]
Hedging activities	13.6	13.8
Net operating loss carryforward	1.0	29.3
Other	(0.1)	(0.3)
Deferred tax assets - noncurrent	14.5	42.8
Deferred income taxes - net noncurrent liability	242.5	201.4
Deferred income taxes - net current asset	$ 1.6	$ 1.8