Comprehensive Income (Details) (USD $) In Millions	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	Dec. 31, 2010
Entity Information [Line Items]
Net income	$ 36.1	$ 27.7	$ 146.3	$ 276.8	$ 210.0	$ 427.7
Other comprehensive income (loss) [Abstract]
Employee benefits	0	(38.0)	0	(12.5)	34.0	35.3
Long-term investment	0	0	0	0.1	0	0.1
Interest rate cash flow hedges	(37.0)	0	(36.2)	0	(36.2)	0
Discontinued operations [Abstract]
Change in unrealized fair value of derivatives held by QEP	0	0	0	(138.7)	0	(151.3)
Income taxes	13.7	14.5	13.4	56.5	0.3	42.9
Net other comprehensive (loss)	(23.3)	(23.5)	(22.8)	(94.6)	(1.9)	(73.0)
Comprehensive income	12.8	4.2	123.5	182.2	208.1	354.7
Discontinued operations, noncontrolling interest	0	0	0	(1.3)	0	(2.2)
Comprehensive income attributable to Questar	12.8	4.2	123.5	180.9	208.1	352.5
Components of Accumulated Other Comprehensive Income (Loss) [Abstract]
Employee benefits	(104.1)	(125.0)	(104.1)	(125.0)	(104.1)	(125.0)	(104.1)
Long-term investment	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Interest rate cash flow hedges	(22.8)	0	(22.8)	0	(22.8)	0	0
Accumulated Other Comprehensive (Loss)	(126.8)	(124.9)	(126.8)	(124.9)	(126.8)	(124.9)	(104.0)
Questar Pipeline [Member]
Entity Information [Line Items]
Net income	18.8	16.3	50.7	49.4	68.7	63.8
Other comprehensive income (loss) [Abstract]
Interest rate cash flow hedges	(37.0)	0	(36.2)	0	(36.2)	0
Discontinued operations [Abstract]
Income taxes	13.7	0	13.4	0	13.4	0
Net other comprehensive (loss)	(23.3)	0	(22.8)	0	(22.8)	0
Comprehensive income	(4.5)	16.3	27.9	49.4	45.9	63.8
Components of Accumulated Other Comprehensive Income (Loss) [Abstract]
Interest rate cash flow hedges	(22.8)	0	(22.8)	0	(22.8)	0	0
Accumulated Other Comprehensive (Loss)	$ (22.8)	$ 0	$ (22.8)	$ 0	$ (22.8)	$ 0	$ 0

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Includes elements of defined-benefit, nonqualified pension, life and health plan obligations. The defined-benefit pension plan is the largest component of the number.

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Accumulated appreciation or loss, net of tax, in value of the total of unsold securities at the end of an accounting period.

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Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's increase (decrease) in deferred hedging gains or losses.

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The total of net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at fiscal year-end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, and unrealized gains and losses on certain investments in debt and equity securities as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the reporting entity. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, but excludes any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

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The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the economic entity, including both controlling (parent) and noncontrolling interests. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, including any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

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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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Gross appreciation or the gross loss in value of the total of unsold available-for-sale securities (except when designated as a fair value hedge or which had been determined to have had an other than temporary decline in fair value below its amortized cost basis) at the end of an accounting period.

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This element represents Other Comprehensive Income (Loss), Net of Tax, for the period. Includes deferred gains (losses) on qualifying hedges, unrealized holding gains (losses) on available-for-sale securities, minimum pension liability, and cumulative translation adjustment.

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Tax effect of the change in accumulated other comprehensive income, that is, the tax effect on items included in other comprehensive income during the period.

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The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

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