Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value Hedging [Member] | Interest Rate Derivative Instrument [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gain	$ 0	$ 0	$ 9,800,000
Fair Value Hedging [Member] | Notes Due 2016, 2.75% [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	0	0	(9,800,000)
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	0	(37,300,000)
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	(500,000)	(400,000)	(100,000)
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 Terminated 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
Date through which reclassifications into earnings from AOCI will take place	2041
Pre-tax net losses expected to be reclassified from AOCI to earnings	$ 500,000
Reclassification from accumulated OCI to income, estimated time to transfer	12 months