Employee Benefits Subsidiary plan participation and Employee Investment Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Subsidiary plan participation and Employee Investment Plan [Line Items]
Percentage of employer contribution match on employees' pre-tax 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 EIP contribution expense amount recognized in the current period	$ 7.3	$ 7.0	$ 5.9
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	38.5	25.8	25.3
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	7.1	5.1	6.2
Questar Gas [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Yearly EIP contribution expense amount recognized in the current period	3.6	3.4	3.9
Questar Gas [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	19.7	13.4	15.4
Questar Gas [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	3.3	2.3	3.6
Questar Pipeline [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Yearly EIP contribution expense amount recognized in the current period	1.4	1.4	1.2
Questar Pipeline [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	7.4	5.0	4.1
Questar Pipeline [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Periodic expense	$ 1.2	$ 1.0	$ 0.8