Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Federal [Abstract]
Current	$ 45.1	$ 69.6	$ 66.5
Deferred	56.3	47.5	18.4
State [Abstract]
Current	5.1	6.9	10.7
Deferred	4.3	2.1	5.9
Deferred investment tax credits recognized	(0.2)	(0.2)	(0.2)
Total income tax expense	$ 110.6	$ 125.9	$ 101.3
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)	1.90%	1.60%	4.00%
Effective Income Tax Rate Reconciliation, Deduction, Qualified Production Activity, Percent	(2.00%)	(1.50%)	(1.50%)
Tax benefits from dividends paid to employee stock plan (in hundredths)	(1.20%)	(1.00%)	(1.00%)
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	(0.10%)	0.00%	0.00%
Other (in hundredths)	1.00%	1.60%	2.10%
Effective income tax rate (in hundredths)	34.60%	35.70%	38.60%
Deferred tax liabilities [Abstract]
Property, plant and equipment	$ 881.8	$ 827.0
Employee benefits	53.6	52.0
Other	0.6	0.5
Deferred tax liabilities - noncurrent	936.0	879.5
Deferred tax assets [Abstract]
Asset retirement obligations	21.3	22.5
Pension and other postretirement benefits	101.0	110.8
Deferred compensation	13.6	15.7
Hedging activities	13.1	13.2
State tax credits	3.4	3.3
Valuation allowance	(2.3)	(1.6)
Deferred tax assets, net of allowance - noncurrent	150.1	163.9
Net deferred income tax liability - noncurrent	785.9	715.6
Deferred income taxes - current [Abstract]
Deferred tax assets - current	14.9	15.7
Deferred tax liabilities - current	8.5	9.9
Net deferred income tax asset - current	6.4	5.8
Questar Gas [Member]
Federal [Abstract]
Current	(16.0)	(11.9)	$ 6.0
Deferred	48.8	42.4	23.5
State [Abstract]
Current	(2.0)	(1.9)	0.6
Deferred	4.2	3.6	2.0
Deferred investment tax credits recognized	(0.2)	(0.2)	(0.2)
Total income tax expense	$ 34.8	$ 32.0	$ 31.9
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%	1.30%	2.10%
Amortization of investment tax credits related to rate-regulated assets (in hundredths)	(0.20%)	(0.20%)	(0.30%)
Other (in hundredths)	(1.10%)	0.60%	0.90%
Effective income tax rate (in hundredths)	35.10%	36.70%	37.70%
Deferred tax liabilities [Abstract]
Property, plant and equipment	$ 403.0	$ 354.4
Employee benefits	28.0	23.5
Other	0.6	0.5
Deferred tax liabilities - noncurrent	431.6	378.4
Deferred tax assets [Abstract]
Deferred compensation	0.9	0.9
Deferred tax assets, net of allowance - noncurrent	0.9	0.9
Net deferred income tax liability - noncurrent	430.7	377.5
Deferred income taxes - current [Abstract]
Deferred tax assets - current	2.5	3.6
Deferred tax liabilities - current	8.5	9.9
Net deferred income tax (liability) - current	6.0	6.3
Questar Pipeline [Member]
Federal [Abstract]
Current	21.7	21.9	$ 20.2
Deferred	10.0	11.6	(15.2)
State [Abstract]
Current	1.0	1.0	1.3
Deferred	0.7	1.1	0.7
Total income tax expense	$ 33.4	$ 35.6	$ 7.0
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)	1.20%	1.50%	8.50%
Other (in hundredths)	(0.30%)	0.50%	2.60%
Effective income tax rate (in hundredths)	35.90%	37.00%	46.10%
Deferred tax liabilities [Abstract]
Property, plant and equipment	$ 254.0	$ 246.1
Employee benefits	12.4	10.8
Deferred tax liabilities - noncurrent	266.4	256.9
Deferred tax assets [Abstract]
Deferred compensation	1.6	2.0
Hedging activities	13.1	13.2
State tax credits	0.7	0.7
Valuation allowance	(0.5)	(0.4)
Deferred tax assets, net of allowance - noncurrent	14.9	15.5
Net deferred income tax liability - noncurrent	251.5	241.4
Deferred income taxes - current [Abstract]
Net deferred income tax asset - current	$ 1.6	$ 1.9