Derivative Instruments and Hedging Activities, Asset and Liability Derivative Instruments Designated as Hedges in the Balance Sheets (Details) (USD $) In Millions	Sep. 30, 2011
Derivatives, Fair Value [Line Items]
Derivative assets	$ 8.2
Derivative liabilities	36.2
Questar Pipeline [Member] \| Cash Flow Hedging [Member] \| Accounts Payable and Accrued Expenses [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	36.2
Fair Value Hedging [Member] \| Prepaid Expenses and Other [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	0.5
Fair Value Hedging [Member] \| Other Noncurrent Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets	$ 7.7

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

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 a

-Clause 1

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- Definition

Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References

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- Definition

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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