Discontinued Operations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Discontinued Operations [Abstract]
Revenues	$ 0	$ 0	$ 0	$ 529.7
Separation costs	0	0	0	14.0
Operating income	0	0	0	127.6
Discontinued operations, net of income taxes	0	0	0	69.5
Discontinued operations, noncontrolling interest	0	0	0	(0.7)
Total Discontinued Operations, Net Of Income Taxes	$ 0	$ 0	$ 0	$ 68.8
Earnings Per Common Share Attributable To Questar
Basic from discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.39
Diluted from discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.39

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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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Amount of operating income or loss attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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

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The amount of net income or loss derived from discontinued operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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