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, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Fair Value Hedging [Member] | Interest Rate Derivative Instruments [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gain (loss)	$ 0	$ 0	$ 9.8	$ 0
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss)	0	0	(9.8)	0
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	(37.3)	0
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