Derivative Instruments and Hedging Activities, Narrative (Details) (Questar Pipeline [Member], Cash Flow Hedging [Member], Forward Starting Interest Rate Swaps [Member], USD $) In Millions	Jun. 30, 2011
Questar Pipeline [Member] \| Cash Flow Hedging [Member] \| Forward Starting Interest Rate Swaps [Member]
Derivative [Line Items]
Fixed-rate debt expected to be issued	$ 250.0

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Principles Board Opinion (APB)
-Number 21
-Paragraph 16, 20

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant