Derivative Instruments and Hedging Activities Additional Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Sep. 30, 2011 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, 2011 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, 2011 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, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Sep. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount							$ 150.0								$ 125.0
Weighted-average fixed interest rate (in hundredths)															2.75%
Derivative, gain on derivative								7.2
Debt instrument, face amount							180.0
Interest payment for terminated derivative instrument							37.3
Ineffectiveness recognized on the fair value hedge									0	0	0	0	0	0
Ineffectiveness recognized on cash flow hedges	0		0
Interest rate cash flow hedge ineffectiveness is immaterial		Ineffectiveness recognized on the cash flow hedges was de minimis		Ineffectiveness recognized on the cash flow hedges was de minimis	Ineffectiveness recognized on the cash flow hedges was de minimis	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			$ 0.4
Reclassification from accumulated OCI to income, estimated time to transfer			12 months
Life of long-term debt associated with cash flow hedges	30 years		30 years		30 years