Schedule of Valuation and Qualifying Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	$ 3.2	$ 5.4	$ 5.8
Amounts charged to expense	2.4	0.5	3.4
Deductions for accounts written off and other	(2.4)	(2.7)	(3.8)
Ending Balance	3.2	3.2	5.4
Questar Pipeline [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	0.3	0.3	0.4
Amounts charged to expense	0	0	(0.1)
Deductions for accounts written off and other	0	0	0
Ending Balance	0.3	0.3	0.3
Questar Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	2.9	5.1	5.4
Amounts charged to expense	2.4	0.5	3.5
Deductions for accounts written off and other	(2.4)	(2.7)	(3.8)
Ending Balance	$ 2.9	$ 2.9	$ 5.1