Rate Regulation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 70.0	$ 79.6
Noncurrent regulatory assets	14.9	25.0
Current regulatory liabilities	6.4	13.4
Noncurrent regulatory liabilities	75.6	69.9
Questar Gas [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	69.8	78.3
Noncurrent regulatory assets	11.9	21.3
Current regulatory liabilities	4.0	12.5
Noncurrent regulatory liabilities	$ 65.6	60.9
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	9.50%
Questar Gas [Member] | Purchased-gas adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 18.9	39.2
Noncurrent regulatory assets	0.0	0.0
Questar Gas [Member] | Energy-efficiency program [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.1	0.0
Noncurrent regulatory assets	0.0	0.0
Current regulatory liabilities	0.0	0.3
Noncurrent regulatory liabilities	0.0	0.0
Questar Gas [Member] | Contract withholding [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	20.3	13.6
Noncurrent regulatory assets	0.0	0.0
Questar Gas [Member] | Deferred cost-of-service gas charges [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	19.5	25.5
Noncurrent regulatory assets	$ 8.1	9.3
Regulatory asset remaining recovery period (in years)	12 months
Regulatory asset amortization period (in years)	18 years
Questar Gas [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 0.0	0.0
Noncurrent regulatory assets	$ 3.8	4.3
Regulatory asset remaining recovery period (in years)	7 years 2 months
Questar Gas [Member] | Pipeline integrity costs [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 6.3	0.0
Noncurrent regulatory assets	0.0	7.7
Amount of allowed yearly recovery through pipeline integrity regulations	7.0
Questar Gas [Member] | CET [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	3.6	0.0
Noncurrent regulatory assets	0.0	0.0
Current regulatory liabilities	0.0	12.1
Noncurrent regulatory liabilities	0.0	0.0
Questar Gas [Member] | Cost of plant removal [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	3.7	0.0
Noncurrent regulatory liabilities	65.5	60.7
Questar Gas [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0.0	0.0
Noncurrent regulatory liabilities	0.1	0.2
Questar Gas [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.1	0.0
Noncurrent regulatory assets	0.0	0.0
Current regulatory liabilities	0.3	0.1
Noncurrent regulatory liabilities	0.0	0.0
Questar Pipeline [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.2	1.3
Noncurrent regulatory assets	3.0	3.7
Current regulatory liabilities	2.4	0.9
Noncurrent regulatory liabilities	10.0	9.0
Questar Pipeline [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.0	0.0
Noncurrent regulatory assets	$ 1.8	2.2
Regulatory asset remaining recovery period (in years)	6 years
Questar Pipeline [Member] | Gas imbalance [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 0.0	1.2
Noncurrent regulatory assets	0.0	0.0
Current regulatory liabilities	2.3	0.8
Noncurrent regulatory liabilities	0.0	0.0
Questar Pipeline [Member] | Revenue sharing [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.2	0.1
Noncurrent regulatory assets	0.0	0.0
Current regulatory liabilities	0.1	0.1
Noncurrent regulatory liabilities	0.0	0.0
Questar Pipeline [Member] | Income taxes recoverable from/refundable to customers [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.0	0.0
Noncurrent regulatory assets	0.0	0.3
Questar Pipeline [Member] | Postretirement medical [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0.0	0.0
Noncurrent regulatory liabilities	10.0	9.0
Questar Pipeline [Member] | Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0.0	0.0
Noncurrent regulatory assets	$ 1.2	$ 1.2