Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value Hedging [Member] | Interest Rate Derivative Instrument [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gain	$ 0	$ 9,800,000	$ 0
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	0	(9,800,000)	0
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Interest Rate Derivative Instrument [Member] | Other Comprehensive Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Deferrals of effective portions	0	(37,300,000)	0
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Interest Rate Derivative Instrument [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Losses reclassified from AOCI into earnings for effective portions	(400,000)	(100,000)	0
Interest Rate Swap [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness recognized on the fair value hedge	0	0	0
Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member] | Questar Pipeline [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Ineffectiveness recognized on the cash flow hedges	0		0
Interest rate cash flow hedge ineffectiveness is immaterial		Ineffectiveness recognized on the cash flow hedges was de minimis
Pre-tax net losses expected to be reclassified from AOCI to earnings	$ 500,000
Date through which reclassifications into earnings from AOCI will take place	2041