Derivatives and Hedging (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2015	Dec. 31, 2011	Dec. 31, 2014	Sep. 30, 2011	Jun. 30, 2011
Derivative [Line Items]
Derivative assets or liabilities outstanding			$ 0.2		$ 0.0
Cash Flow Hedging [Member] | Questar Pipeline [Member] | Forward Starting Interest Rate Swaps Terminated Q4 2011 [Member]
Derivative [Line Items]
Notional amount						$ 150.0
Debt Instrument, Face Amount		$ 180.0		$ 180.0
Derivative, loss on derivative		$ 37.3
Life of long-term debt associated with cash flow hedges				30 years
Date through which reclassifications into earnings from AOCI will take place			2041
Pre-tax net losses expected to be reclassified from AOCI to earnings			$ 0.6
Reclassification from accumulated OCI to income, estimated time to transfer			12 months
Fair Value Hedging [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount							$ 125.0
Weighted-average fixed interest rate (in hundredths)							2.75%
Derivative, gain on derivative	$ 7.2