Operations by Line of Business (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Operations by Line of Business [Abstract]
Number of complementary lines of business (in lines)			3
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	$ 146.7	$ 145.2	$ 839.6	$ 760.9	$ 1,202.3	$ 1,104.9
Revenues from affiliated companies	83.3	76.8	244.4	235.2	324.5	312.7
Operating income (loss)	63.8	54.4	260.3	235.5	368.1	335.4
Income (loss) from continuing operations	36.1	27.7	146.3	128.6	210.0	184.0
Wexpro [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	6.9	6.1	23.1	16.6	31.1	23.4
Revenues from affiliated companies	64.4	58.4	186.4	179.1	247.5	237.6
Operating income (loss)	37.7	32.8	108.3	98.9	143.1	133.1
Income (loss) from continuing operations	25.6	22.2	71.6	65.4	94.3	86.9
Questar Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	50.0	49.3	148.8	145.2	200.8	190.0
Revenues from affiliated companies	18.5	18.2	55.8	55.5	74.3	74.1
Operating income (loss)	33.9	31.9	94.9	96.7	129.0	124.5
Income (loss) from continuing operations	18.8	16.3	50.7	49.4	68.7	63.8
Questar Gas [Member]
Segment Reporting Information [Line Items]
Revenues from unaffiliated customers	89.8	89.8	667.7	599.1	970.4	891.5
Revenues from affiliated companies	0.4	0.2	2.2	0.6	2.7	1.0
Operating income (loss)	(7.9)	(10.3)	56.6	49.4	95.8	85.9
Income (loss) from continuing operations	(7.9)	(9.1)	25.9	21.8	48.0	41.7
Corporate [Member]
Segment Reporting Information [Line Items]
Operating income (loss)	0.1	0	0.5	(9.5)	0.2	(8.1)
Income (loss) from continuing operations	$ (0.4)	$ (1.7)	$ (1.9)	$ (8.0)	$ (1.0)	$ (8.4)

The number of complementary lines of business operated by the company.

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- Definition

This element represents the income or loss from continuing operations attributable to the reporting entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items and cumulative effects of changes in accounting principles, but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests, if any.

+ References

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

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 29

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

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 28

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph 38

-Subparagraph b(1)

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The net result for the period of deducting operating expenses from operating revenues.

+ Details

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- Definition

Amount of revenue from transactions with other operating segments of the same entity. Must be disclosed if the amount (a) is included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) is otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph b

+ Details

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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

Amount of revenue from external customers for the reportable segment. Must be disclosed if the amounts (a) is included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) is otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph a

+ Details

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