Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Federal [Abstract]
Current	$ 69.6	$ 66.5	$ 1.3
Deferred	47.5	18.4	112.0
State [Abstract]
Current	6.9	10.7	0
Deferred	2.1	5.9	3.6
Deferred investment tax credits recognized	(0.2)	(0.2)	(0.4)
Total income tax expense	125.9	101.3	116.5
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)	1.60%	4.00%	1.00%
Tax benefits from dividends paid to employee stock plan (in hundredths)	(1.00%)	(1.00%)	(0.50%)
Other (in hundredths)	0.10%	0.60%	0.00%
Effective income tax rate (in hundredths)	35.70%	38.60%	35.50%
Deferred tax liabilities [Abstract]
Property, plant and equipment	827.0	797.5
Employee benefits	52.0	30.2
Other	0.5	4.9
Deferred tax liabilities - noncurrent	879.5	832.6
Deferred tax assets [Abstract]
Asset retirement obligations	22.5	22.7
Pension and other postretirement benefits	110.8	73.7
Deferred compensation	15.7	14.5
Hedging activities	13.2	13.4
State tax credits	3.3	3.4
Valuation allowance	(1.6)	(2.3)
Deferred tax assets, net of allowance - noncurrent	163.9	125.4
Net deferred income tax liability - noncurrent	715.6	707.2
Deferred income taxes - current [Abstract]
Deferred tax assets - current	15.7	10.1
Deferred tax liabilities - current	9.9	0.4
Net deferred income tax asset - current	5.8	9.7
Questar Gas [Member]
Federal [Abstract]
Current	(11.9)	6.0	(18.7)
Deferred	42.4	23.5	45.1
State [Abstract]
Current	(1.9)	0.6	0
Deferred	3.6	2.0	1.2
Deferred investment tax credits recognized	(0.2)	(0.2)	(0.4)
Total income tax expense	32.0	31.9	27.2
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)	1.30%	2.10%	1.00%
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	(0.20%)	(0.30%)	(0.40%)
Other (in hundredths)	0.60%	0.90%	1.00%
Effective income tax rate (in hundredths)	36.70%	37.70%	36.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	354.4	322.6
Employee benefits	23.5	14.0
Other	0.5	4.9
Deferred tax liabilities - noncurrent	378.4	341.5
Deferred tax assets [Abstract]
Deferred compensation	0.9	0.8
Deferred tax assets, net of allowance - noncurrent	0.9	0.8
Net deferred income tax liability - noncurrent	377.5	340.7
Deferred income taxes - current [Abstract]
Deferred tax assets - current	3.6	3.3
Deferred tax liabilities - current	9.9	0.4
Net deferred income tax asset - current	0	2.9
Net deferred income tax (liability) - current	(6.3)	0
Questar Pipeline [Member]
Federal [Abstract]
Current	21.9	20.2	(5.4)
Deferred	11.6	(15.2)	40.4
State [Abstract]
Current	1.0	1.3	1.6
Deferred	1.1	0.7	0.7
Total income tax expense	35.6	7.0	37.3
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)	1.50%	8.50%	1.60%
Other (in hundredths)	0.50%	2.60%	0.00%
Effective income tax rate (in hundredths)	37.00%	46.10%	36.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	246.1	237.1
Employee benefits	10.8	7.4
Deferred tax liabilities - noncurrent	256.9	244.5
Deferred tax assets [Abstract]
Deferred compensation	2.0	1.8
Hedging activities	13.2	13.4
State tax credits	0.7	0.7
Valuation allowance	(0.4)	0
Deferred tax assets, net of allowance - noncurrent	15.5	15.9
Net deferred income tax liability - noncurrent	241.4	228.6
Deferred income taxes - current [Abstract]
Net deferred income tax asset - current	$ 1.9	$ 1.8