Schedule of Valuation and Qualifying Accounts (Details) (Allowance for Bad Debt [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	$ 3.2	$ 3.2	$ 5.4
Amounts charged to expense	1.2	2.4	0.5
Deductions for accounts written off and other	(1.3)	(2.4)	(2.7)
Ending Balance	3.1	3.2	3.2
Questar Gas [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	2.9	2.9	5.1
Amounts charged to expense	1.1	2.4	0.5
Deductions for accounts written off and other	(1.2)	(2.4)	(2.7)
Ending Balance	2.8	2.9	2.9
Questar Pipeline [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	0.3	0.3	0.3
Amounts charged to expense	0.1	0	0
Deductions for accounts written off and other	(0.1)	0	0
Ending Balance	$ 0.3	$ 0.3	$ 0.3