Rate Regulation (Details) (USD $) In Millions, unless otherwise specified									12 Months Ended											12 Months Ended										12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Questar Pipeline [Member]	Dec. 31, 2010 Questar Pipeline [Member]	Dec. 31, 2011 Questar Pipeline [Member] Gas imbalance [Member]	Dec. 31, 2010 Questar Pipeline [Member] Gas imbalance [Member]	Dec. 31, 2011 Questar Pipeline [Member] Revenue sharing [Member]	Dec. 31, 2010 Questar Pipeline [Member] Revenue sharing [Member]	Dec. 31, 2011 Questar Pipeline [Member] Cost of reacquired debt [Member]	Dec. 31, 2010 Questar Pipeline [Member] Cost of reacquired debt [Member]	Dec. 31, 2011 Questar Pipeline [Member] Income taxes recoverable from customers [Member]	Dec. 31, 2010 Questar Pipeline [Member] Income taxes recoverable from customers [Member]	Dec. 31, 2011 Questar Pipeline [Member] Other [Member]	Dec. 31, 2010 Questar Pipeline [Member] Other [Member]	Dec. 31, 2011 Questar Pipeline [Member] Postretirement medical [Member]	Dec. 31, 2010 Questar Pipeline [Member] Postretirement medical [Member]	Dec. 31, 2011 Questar Gas [Member]	Dec. 31, 2010 Questar Gas [Member]	Aug. 01, 2010 Questar Gas [Member]	Dec. 31, 2011 Questar Gas [Member] Cost of reacquired debt [Member]	Dec. 31, 2010 Questar Gas [Member] Cost of reacquired debt [Member]	Dec. 31, 2011 Questar Gas [Member] Income taxes recoverable from customers [Member]	Dec. 31, 2010 Questar Gas [Member] Income taxes recoverable from customers [Member]	Dec. 31, 2011 Questar Gas [Member] Other [Member]	Dec. 31, 2010 Questar Gas [Member] Other [Member]	Dec. 31, 2011 Questar Gas [Member] DSM [Member]	Dec. 31, 2010 Questar Gas [Member] DSM [Member]	Dec. 31, 2011 Questar Gas [Member] Purchased-gas adjustment [Member]	Dec. 31, 2010 Questar Gas [Member] Purchased-gas adjustment [Member]	Dec. 31, 2011 Questar Gas [Member] Deferred production taxes [Member]	Dec. 31, 2010 Questar Gas [Member] Deferred production taxes [Member]	Dec. 31, 2011 Questar Gas [Member] Pipeline integrity costs [Member]	Dec. 31, 2010 Questar Gas [Member] Pipeline integrity costs [Member]	Dec. 31, 2011 Questar Gas [Member] ARO cost-of-service gas wells [Member]	Dec. 31, 2010 Questar Gas [Member] ARO cost-of-service gas wells [Member]	Dec. 31, 2011 Questar Gas [Member] Conservation enabling tariff [Member]	Dec. 31, 2010 Questar Gas [Member] Conservation enabling tariff [Member]
Regulatory Asset Liabilities [Line Items]
Current regulatory assets	$ 31.7	$ 53.5	$ 5.2	$ 0.8	$ 2.8	$ 0.8	$ 2.4	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0			$ 26.5	$ 52.7		$ 0	$ 0					$ 24.2	$ 39.7	$ 0	$ 10.7	$ 2.3	$ 2.3	$ 0	$ 0	$ 0	$ 0
Noncurrent regulatory assets	21.0	21.7	5.9	6.7	0	0	0	0	3.3	3.7	1.2	1.6	1.4	1.4			15.1	15.0		5.9	6.4					0	0	0	0	0	0	6.5	5.6	2.7	3.0
Current regulatory liabilities	15.4	6.0	1.2	3.2	0.9	3.1	0.3	0.1							0	0	14.2	2.8				0	0	0.2	0			10.8	0							3.2	2.8
Noncurrent Regulatory Liabilities	8.1	7.8	7.4	6.9	0	0	0	0							7.4	6.9	0.7	0.9				0.7	0.9	0	0			0	0							0	0
Weighted-average life of reacquired debt									9 years											11 years
Amount of allowed yearly recover through pipeline integrity regulations																																4.4
Regulatory asset amortization period																																		18 years
Recording period for production taxes on cost-of-service gas production																														12 months
Rate Changes [Abstract]
Original authorized return on equity in Utah																			10.00%
Current authorized return on equity in Utah																			10.35%
Amount of increase in customer rates in Utah																			5.0
Amount of requested increase in customer rates in Wyoming																	$ 1.0
Requested return on equity in Wyoming																	10.25%