Derivative Instruments and Hedging Activities, Effects on Statements of Income and AOCI (Details) (Fair Value Hedging [Member], USD $) In Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2011
Questar Pipeline [Member] \| Interest Rate Derivative Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized on derivatives in accumulated other comprehensive loss (effective portion)	$ (37.0)	$ (36.2)	$ (36.2)
Interest Expense [Member] \| Interest Rate Derivative Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net (losses) gains on derivative instruments	5.2	9.1	9.1
Interest Expense [Member] \| Notes Due 2016, 2.75% [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net (losses) gains on derivative instruments	$ (5.2)	$ (9.1)	$ (9.1)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44C

-Subparagraph b

+ Details

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

+ Details

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