Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Leases [Abstract]
Expiration date of lease on current headquarters building	Apr. 30, 2012
Minimum future payments of long-term operating leases for current year	$ 3.4
Rental expense	3.2	3.4	3.5
New Headquarters building lease term	17 years
Annual initial lease payments for new headquarters building	3.2
Wexpro [Member] | Well-drilling commitment [Member]
Long-term Purchase Commitment [Line Items]
2012	2.8
Questar Gas [Member]
Long-term Purchase Commitment [Line Items]
Minimum gas supply provided by cost-of-service reserves (in hundredths)	40.00%
Maximum gas supply provided by cost-of-service reserves (in hundredths)	50.00%
Natural gas purchased under agreements	221.0	245.2	225.3
Questar Gas [Member] | Gas purchase agreements [Member]
Long-term Purchase Commitment [Line Items]
2012	36.2
2013	15.2
2014	15.2
2015	15.2
2016	15.3
Questar Gas [Member] | Transportation and underground storage commitments - Related party [Member]
Long-term Purchase Commitment [Line Items]
2012	69.9
2013	68.0
2014	64.5
2015	64.5
2016	64.5
Questar Gas [Member] | Transportation and underground storage commitments - Third party [Member]
Long-term Purchase Commitment [Line Items]
2012	28.8
2013	28.8
2014	28.8
2015	28.8
2016	$ 28.8