Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair Value Hedging [Member] | Interest Rate Derivative Instruments [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain	$ 0	$ 3.9	$ 0	$ 3.9	$ 5.9	$ 3.9
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	0	(3.9)	0	(3.9)	(5.9)	(3.9)
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	0.8	0	0.8	(38.1)	0.8
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	$ (0.1)	$ 0	$ (0.2)	$ 0	$ (0.3)	$ 0