Employee Benefits Subsidiary plan participation and Employee Investment Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Subsidiary plan participation and Employee Investment Plan [Line Items]
Percentage of employer contribution match on employees' pretax purchase of common stock (in hundredths)	100.00%
The maximum percentage of an employees' qualifying earnings used to purchase pre-tax purchases of common stock (in hundredths)	6.00%
Yearly contribution expense amount recognized in the current period	$ 7.0	$ 5.9	$ 5.2
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	25.8	25.3	24.0
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	5.1	6.2	6.7
Questar Pipeline [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Yearly contribution expense amount recognized in the current period	1.4	1.2	1.1
Questar Pipeline [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	5.0	4.1	3.8
Questar Pipeline [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	1.0	0.8	0.3
Questar Gas [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Yearly contribution expense amount recognized in the current period	3.4	3.9	3.3
Questar Gas [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	13.4	15.4	13.3
Questar Gas [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	$ 2.3	$ 3.6	$ 3.8