Derivative Instruments and Hedging Activities, Additional Information (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011
Questar Pipeline [Member] | Cash Flow Hedging [Member] | Forward Starting Interest Rate Swaps [Member]
Derivative [Line Items]
Maturity date	2011
Weighted-average fixed interest rate (in hundredths)	403.00%	[1]
Notional amount	$ 100.0
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Maturity date	2016
Weighted-average fixed interest rate (in hundredths)	275.00%	[1]
Notional amount	$ 125.0

[1] Floating rates are based on the 3- and 6-month U.S. dollar LIBOR.