Derivatives and Hedging (Details) (Cash Flow Hedging [Member], Questar Pipeline [Member], Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member], USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2011
Cash Flow Hedging [Member] | Questar Pipeline [Member] | Forward Starting Interest Rate Swaps Terminating Q4 2011 [Member]
Derivative [Line Items]
Notional amount		$ 150.0
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
Pre-tax net losses expected to be reclassified from AOCI to Consolidated statement of income	$ 0.6
Reclassification from accumulated OCI to income, time to transfer	12 months