Discontinued Operations (Details) (USD $) In Millions, except Per Share data	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
Discontinued Operations [Abstract]
Discontinued operations, net of income taxes	$ 0	$ 0	$ 0	$ 148.2	$ 0	$ 243.7
Discontinued operations, noncontrolling interest	0	0	0	1.3	0	2.2
Total discontinued operations, net of income taxes	0	0	0	146.9	0	241.5
Earnings Per Common Share Attributable to Questar [Abstract]
Basic from discontinued operations (in dollars per share)	$ 0	$ 0	$ 0	$ 0.84	$ 0	$ 1.39
Diluted from discontinued operations (in dollars per share)	$ 0	$ 0	$ 0	$ 0.83	$ 0	$ 1.36
QEP operations [Member]
Discontinued Operations [Abstract]
Revenues	0	0	0	1,109.8		1,669.6
Separation costs	0	0	0	14.0		14.0
Operating income	0	0	0	270.5		434.9
Discontinued operations, net of income taxes	0	0	0	148.2		243.7
Discontinued operations, noncontrolling interest	0	0	0	(1.3)		(2.2)
Total discontinued operations, net of income taxes	$ 0	$ 0	$ 0	$ 146.9		$ 241.5
Earnings Per Common Share Attributable to Questar [Abstract]
Basic from discontinued operations (in dollars per share)	$ 0	$ 0	$ 0	$ 0.84		$ 1.39
Diluted from discontinued operations (in dollars per share)	$ 0	$ 0	$ 0	$ 0.83		$ 1.36

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Amount of severance costs attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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Overall income (loss) from a disposal group that is classified as a component of the entity, before income tax, reported as a separate component of income before extraordinary items and the cumulative effect of accounting changes. Includes the following (before income tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

+ References

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Amount of sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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This element represents the overall income (loss) from a disposal group that is classified as a component of the entity, net of income tax, reported as a separate component of income before extraordinary items and the cumulative effect of accounting changes before deduction or consideration of the amount which may be allocable to noncontrolling interests, if any. Includes the following (net of tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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This element represents the overall income (loss) from a disposal group that is classified as a component of the entity, net of income tax, reported as a separate component of income before extraordinary items and the cumulative effect of accounting changes, which is apportioned to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This item includes the following (net of tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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This element represents the overall income (loss) from a disposal group apportioned to the parent that is classified as a component of the entity, net of income tax, reported as a separate component of income before extraordinary items and the cumulative effect of accounting changes after deduction or consideration of the amount which may be allocable to noncontrolling interests, if any. Includes the following (net of tax): income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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The amount of income (loss) from disposition of discontinued operations, net of related tax effect, per each share of common stock outstanding during the reporting period.

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-Publisher FASB

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The amount of income (loss) from discontinued operations, net of related tax effect, per each diluted share of common stock outstanding during the reporting period.

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