Employee Benefits Subsidiary plan participation and Employee Investment Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 7.2	$ 7.3	$ 7.0
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	36.4	38.5	25.8
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	5.3	6.6	4.8
Questar Gas [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Yearly EIP contribution expense amount recognized in the current period	3.4	3.6	3.4
Questar Gas [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	18.1	19.7	13.4
Share of total defined benefit plan contributions	29.6	39.4	27.1
Questar Gas [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	2.4	3.3	2.3
Share of total defined benefit plan contributions	2.0	1.9	2.2
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	1.4
Questar Pipeline [Member] | Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	6.7	7.4	5.0
Share of total defined benefit plan contributions	11.2	15.1	8.7
Questar Pipeline [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and Employee Investment Plan [Line Items]
Net periodic cost	0.2	1.2	1.0
Share of total defined benefit plan contributions	$ 0.7	$ 0.6	$ 0.2