Derivative Instruments and Hedging Activities Additional Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended		1 Months Ended
	Dec. 31, 2013 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminated Q4 2011 [Member]	Dec. 31, 2011 Cash Flow Hedging [Member] Questar Pipeline [Member] Forward Starting Interest Rate Swaps Terminated Q4 2011 [Member]	Mar. 31, 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
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
Life of long-term debt associated with cash flow hedges	30 years