Rate Regulation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	$ 46.7	$ 31.7
Noncurrent regulatory assets	21.6	21.0
Current regulatory liabilities	5.8	15.4
Noncurrent Regulatory Liabilities	8.2	8.1
Questar Gas [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	41.0	26.5
Noncurrent regulatory assets	16.4	15.1
Current regulatory liabilities	4.3	14.2
Noncurrent Regulatory Liabilities	0.5	0.7
Rate Changes [Abstract]
Current authorized return on equity in Utah	10.35%
Amount of increase in customer rates in Wyoming	0.6
Current authorized return on equity in Wyoming	9.16%
Questar Gas [Member] | Purchased-gas adjustment [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	15.2	0
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0	10.8
Noncurrent Regulatory Liabilities	0	0
Questar Gas [Member] | DSM [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	11.0	24.2
Noncurrent regulatory assets	0	0
Questar Gas [Member] | Deferred production taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	5.0	2.3
Noncurrent regulatory assets	0	0
Recording period for production taxes and royalties on cost-of-service gas production	12 months
Questar Gas [Member] | Deferred royalties [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	6.3	0
Noncurrent regulatory assets	0	0
Questar Gas [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	5.3	5.9
Weighted-average life of reacquired debt	10 years
Questar Gas [Member] | Pipeline integrity costs [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	8.7	6.5
Amount of allowed yearly recovery through pipeline integrity regulations	4.4
Questar Gas [Member] | ARO cost-of-service gas wells [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	2.4	2.7
Regulatory asset amortization period	P18Y
Questar Gas [Member] | CET [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	4.2	3.2
Noncurrent Regulatory Liabilities	0	0
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	0.5	0.7
Questar Gas [Member] | Other [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	3.5	0
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.1	0.2
Noncurrent Regulatory Liabilities	0	0
Questar Pipeline [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	5.7	5.2
Noncurrent regulatory assets	5.2	5.9
Current regulatory liabilities	1.5	1.2
Noncurrent Regulatory Liabilities	7.7	7.4
Questar Pipeline [Member] | Cost of reacquired debt [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	3.0	3.3
Weighted-average life of reacquired debt	8 years
Questar Pipeline [Member] | Gas imbalance [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	4.4	2.8
Noncurrent regulatory assets	0	0
Current regulatory liabilities	1.4	0.9
Noncurrent Regulatory Liabilities	0	0
Questar Pipeline [Member] | Revenue sharing [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	1.3	2.4
Noncurrent regulatory assets	0	0
Current regulatory liabilities	0.1	0.3
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	0.9	1.2
Questar Pipeline [Member] | Postretirement medical [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory liabilities	0	0
Noncurrent Regulatory Liabilities	7.7	7.4
Questar Pipeline [Member] | Other [Member]
Regulatory Assets and Liabilities [Line Items]
Current regulatory assets	0	0
Noncurrent regulatory assets	$ 1.3	$ 1.4