Derivative Instruments and Hedging Activities Additional Information (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2013 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2013 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Mar. 31, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2013 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2013 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2013 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount							$ 150,000,000.0								$ 125,000,000
Weighted-average fixed interest rate (in hundredths)															2.75%
Derivative, gain on derivative								7,200,000
Debt Instrument, Face Amount							180,000,000.0
Interest payment for terminated derivative instrument							37,300,000
Life of long-term debt associated with cash flow hedges							30 years
Ineffectiveness recognized on the fair value hedge									0	0	0	0	0	0
Ineffectiveness recognized on cash flow hedges	0	0	0	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 Consolidated statement of income			$ 500,000
Reclassification from accumulated OCI to income, estimated time to transfer			12 months