Derivative Instruments and Hedging Activities Additional Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended			1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Jun. 30, 2011 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Jun. 30, 2011 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Jun. 30, 2012 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]	Jun. 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]	Jun. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 30, 2012 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 30, 2011 Fair Value Hedging [Member] Interest Rate Swap [Member]	Jun. 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							$ 150.0		$ 125.0		$ 125.0		$ 125.0
Weighted-average fixed interest rate (in hundredths)									2.75%		2.75%		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	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			$ 0.5
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