Derivative Instruments and Hedging Activities, Additional Information (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011
Derivative [Line Items]
Reclassification from AOCI to income, estimated time to transfer (in months)	12M
Pre-tax net losses (gain) expected to be reclassified from AOCI to Consolidated statement of income	$ 0.4
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)	3.91%	[1]
Notional amount	150.0
Interest payment for terminated derivative instrument	29.1
Fixed interest rate on terminated derivative instrument (in hundredths)	2.97%
Questar Pipeline [Member] | Cash Flow Hedging [Member] | New Forward Starting Interest Rate Swaps [Member]
Derivative [Line Items]
Maturity date	2011
Weighted-average fixed interest rate (in hundredths)	3.00%
Notional amount	150.0
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Maturity date	2016
Weighted-average fixed interest rate (in hundredths)	2.75%	[1]
Notional amount	$ 125.0

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