Derivative Instruments and Hedging Activities Effects on Statements of Income and OCI (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value Hedging [Member] \| Interest Rate Derivative Instrument [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gain	$ 9.8	$ 0	$ 0
Fair Value Hedging [Member] \| Notes Due 2016, 2.75% [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized loss	(9.8)	0	0
Questar Pipeline [Member] \| Cash Flow Hedging [Member] \| Interest Rate Derivative Instrument [Member] \| Other Comprehensive Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Deferrals of effective portions	(37.3)	0	0
Questar Pipeline [Member] \| Cash Flow Hedging [Member] \| Interest Rate Derivative Instrument [Member] \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Losses reclassified from AOCI into earnings for effective portions	$ (0.1)	$ 0	$ 0

The amount of net gains (losses) on a cash flow hedge reclassified in the period to earnings from accumulated other comprehensive income.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph b(1)

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- Definition

Amount of the increase or decrease in fair value during the period of the hedged item in a fair value hedge. Recognized in earnings and is offset by the gain (loss) on the hedging instrument to the extent that the fair value hedge is determined to be effective.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 45

-Subparagraph a(1)

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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

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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

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