Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Fair Value Hedging [Member] | Interest Rate Derivative Instruments [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain	$ 0	$ 0	$ 0	$ 9.8
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	0	0	0	(9.8)
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	0	(37.3)
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)	$ (0.1)	$ (0.5)	$ (0.2)