Rate Regulation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 79.6	$ 35.8
Noncurrent regulatory assets	25.0	20.5
Current regulatory liabilities	13.4	14.1
Noncurrent regulatory liabilities	69.9	61.2
Questar Gas [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	78.3	30.2
Noncurrent regulatory assets	21.3	16.1
Current regulatory liabilities	12.5	11.1
Noncurrent regulatory liabilities	60.9	53.0
Rate Changes [Abstract]
Current authorized return on equity in Utah	9.85%
Amount of increase in annual revenues in Utah	7.6
Amount of increase in customer rates in Wyoming	1.5
Current authorized return on equity in Wyoming	9.16%
Future Authorized Return on Equity in Wyoming	9.50%
Questar Gas [Member] | Purchased-gas adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	39.2	0
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0	7.4
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Demand-side management [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	11.2
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.3	0
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Contract withholding [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	13.6	8.1
Noncurrent regulatory assets	0	0
Questar Gas [Member] | Deferred cost-of-service gas charges [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	25.5	10.9
Noncurrent regulatory assets	9.3	2.0
Regulatory asset remaining recovery period (in years)	12 months
Regulatory asset amortization period (in years)	18-year
Questar Gas [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	4.3	4.8
Regulatory asset remaining recovery period (in years)	8 years
Questar Gas [Member] | Pipeline integrity costs [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	7.7	9.3
Amount of allowed yearly recovery through pipeline integrity regulations	7.0
Questar Gas [Member] | CET [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	12.1	3.3
Noncurrent regulatory liabilities	0	0
Questar Gas [Member] | Cost of plant removal [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	60.7	52.7
Questar Gas [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	0.2	0.3
Questar Gas [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0.1	0.4
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.3	5.6
Noncurrent regulatory assets	3.7	4.4
Current regulatory liabilities	0.9	3.0
Noncurrent regulatory liabilities	9.0	8.2
Questar Pipeline [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	2.2	2.6
Regulatory asset remaining recovery period (in years)	6 years
Questar Pipeline [Member] | Gas imbalance [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.2	5.3
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.8	3.0
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member] | Revenue sharing [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.1	0.3
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.1	0
Noncurrent regulatory liabilities	0	0
Questar Pipeline [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	0.3	0.6
Questar Pipeline [Member] | Postretirement medical [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent regulatory liabilities	9.0	8.2
Questar Pipeline [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	$ 1.2	$ 1.2