Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Federal [Abstract]
Current	$ 66.5	$ 1.3	$ 0
Deferred	18.4	112.0	110.8
State [Abstract]
Current	10.7	0	(1.3)
Deferred	5.9	3.6	7.3
Deferred investment tax credits recognized	(0.2)	(0.4)	(0.4)
Total income tax expense	101.3	116.5	116.4
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)	4.00%	1.00%	1.30%
Tax benefits from dividends paid to employee stock plan (in hundredths)	(1.00%)	(0.50%)	(0.50%)
Other (in hundredths)	0.60%	0.00%	0.10%
Effective income tax rate (in hundredths)	38.60%	35.50%	35.90%
Deferred tax liabilities [Abstract]
Property, plant and equipment	797.5	767.0
Employee benefits	30.2	33.9
Other	4.9	3.2
Deferred tax liabilities - noncurrent	832.6	804.1
Deferred tax assets [Abstract]
Asset retirement obligations	22.7	23.2
Pension and other postretirement benefits	73.7	138.3
Deferred compensation	14.5	8.6
Hedging activities	13.4	13.6
State tax credits	3.4	3.4
Net operating loss carryforward	0	15.8
Valuation allowance	(2.3)	(2.2)
Deferred tax assets, net of allowance - noncurrent	125.4	200.7
Net deferred income tax liability - noncurrent	707.2	603.4
Deferred income taxes - current [Abstract]
Deferred tax assets - current	10.1	17.6
Deferred tax liabilities - current	0.4	4.6
Net deferred income tax asset - current	9.7	13.0
Questar Gas [Member]
Federal [Abstract]
Current	6.0	(18.7)	0
Deferred	23.5	45.1	26.3
State [Abstract]
Current	0.6	0	(1.6)
Deferred	2.0	1.2	3.2
Deferred investment tax credits recognized	(0.2)	(0.4)	(0.4)
Total income tax expense	31.9	27.2	27.5
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)	2.10%	1.00%	1.40%
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	(0.30%)	(0.40%)	(0.50%)
Other (in hundredths)	0.90%	1.00%	1.50%
Effective income tax rate (in hundredths)	37.70%	36.60%	37.40%
Deferred tax liabilities [Abstract]
Property, plant and equipment	322.6	293.0
Employee benefits	14.0	17.2
Other	4.9	3.4
Deferred tax liabilities - noncurrent	341.5	313.6
Deferred tax assets [Abstract]
Deferred compensation	0.8	0.9
Net operating loss carryforward	0	11.1
Deferred tax assets, net of allowance - noncurrent	0.8	12.0
Net deferred income tax liability - noncurrent	340.7	301.6
Deferred income taxes - current [Abstract]
Deferred tax assets - current	3.3	6.1
Deferred tax liabilities - current	0.4	4.6
Net deferred income tax asset - current	2.9	1.5
Questar Pipeline [Member]
Federal [Abstract]
Current	20.2	(5.4)	0
Deferred	(15.2)	40.4	37.0
State [Abstract]
Current	1.3	1.6	(0.7)
Deferred	0.7	0.7	1.9
Total income tax expense	7.0	37.3	38.2
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)	8.50%	1.60%	1.00%
Other (in hundredths)	2.60%	0.00%	0.00%
Effective income tax rate (in hundredths)	46.10%	36.60%	36.00%
Deferred tax liabilities [Abstract]
Property, plant and equipment	237.1	250.8
Employee benefits	7.4	8.4
Deferred tax liabilities - noncurrent	244.5	259.2
Deferred tax assets [Abstract]
Deferred compensation	1.8	1.5
Hedging activities	13.4	13.6
State tax credits	0.7	0.7
Net operating loss carryforward	0	1.0
Other	0	(0.1)
Deferred tax assets, net of allowance - noncurrent	15.9	16.7
Net deferred income tax liability - noncurrent	228.6	242.5
Deferred income taxes - current [Abstract]
Net deferred income tax asset - current	$ 1.8	$ 1.6