Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value Hedging [Member] | Interest Rate Derivative Instruments [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain	$ 0	$ 5.2	$ 0	$ 9.1	$ 0.7	$ 9.1
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	0	(5.2)	0	(9.1)	(0.7)	(9.1)
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Interest Rate Derivative Instruments [Member] | Other Comprehensive Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Deferrals of effective portions	0	(37.0)	0	(36.2)	(1.1)	(36.2)
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Interest Rate Derivative Instruments [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Losses reclassified from AOCI into earnings for effective portions	$ (0.2)	$ 0	$ (0.4)	$ 0	$ (0.5)	$ 0