Basis of Presentation of Interim Financial Statements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Cost of Sales [Abstract]
Other cost of sales	$ 0.3us-gaap_CostOfNaturalGasPurchases	$ 0.1us-gaap_CostOfNaturalGasPurchases	$ 1.1us-gaap_CostOfNaturalGasPurchases	$ 0.2us-gaap_CostOfNaturalGasPurchases
Total cost of sales	131.8us-gaap_CostOfGoodsSoldOilAndGas	148.2us-gaap_CostOfGoodsSoldOilAndGas	169.9us-gaap_CostOfGoodsSoldOilAndGas	234.4us-gaap_CostOfGoodsSoldOilAndGas
Questar Gas [Member]
Payments for Questar Gas Acquisition	11.4		11.4
Additional Customers from Questar Gas Acquisition	6,500
Cost of Sales [Abstract]
Gas purchases	41.0	92.7	84.8	176.0
Operator service fee	84.8	89.1	345.4	310.9
Transportation and storage	22.2	22.1	79.7	80.2
Gathering	5.4	4.9	21.5	19.1
Royalties	11.9	17.3	54.7	50.5
Storage (injection) withdrawal, net	30.9	33.0	(3.2)	3.4
Purchased-gas account adjustment	36.2	(5.6)	(4.0)	(30.3)
Other	1.2	1.1	4.9	4.8
Total Questar Gas cost of natural gas sold	233.6	254.6	583.8	614.6
Elimination of Questar Gas cost of natural gas sold - affiliated companies	(104.0)	(108.2)	(419.2)	(387.0)
Total Questar Gas cost of natural gas sold - unaffiliated parties	129.6	146.4	164.6	227.6
Questar Pipeline [Member]
Ownership percentage in unconsolidated affiliate (in hundredths)	50.00%		50.00%
Cost of Sales [Abstract]
Total Questar Pipeline cost of sales	$ 1.9	$ 1.7	$ 4.2	$ 6.6

Additional Customers from Questar Gas Acquisition

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- Definition

Cost of oil and gas purchased from and transported by affiliated companies during the reporting period.

+ References

No definition available.

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Cost of oil and gas purchased from unaffiliated parties during the reporting period.

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No definition available.

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Cost of natural gas purchased from third parties. Excludes transportation, gathering and storage costs.

+ References

No definition available.

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- Definition

Natural gas gathering costs charged by third parties.

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No definition available.

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- Definition

Operator service fee charged to Questar Gas by Wexpro for the development and operation of cost-of-service natural gas reserves.

+ References

No definition available.

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- Definition

Other miscellaneous components of Questar Gas cost of natural gas sold.

+ References

No definition available.

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- Definition

Portion of gas purchase costs that represents amounts paid to Wexpro to reimburse them for royalties on natural gas and other products. Royalties are billed to Questar Gas by Wexpro separate from the operator service fee.

+ References

No definition available.

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- Definition

Represents the offset to inventory entries when natural gas is physically injected into or withdrawn from storage facilities. As inventory is increased (debited), this offset component of gas costs is credited and vice versa.

+ References

No definition available.

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Natural gas transportation and storage costs, predominantly consists of amounts charged to Questar Gas by Questar Pipeline.

+ References

No definition available.

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- Definition

Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

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-Section 55

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- Definition

The cost of generation, transmission, and distribution of [natural] gas, which are regulated by a federal, state, or local government or agency.

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- Definition

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 2

-Article 5

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-Subparagraph (SX 210.5-03.2(a),(d))

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- Definition

Total costs related to oil and gas produced and sold during the reporting period.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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Cost of natural gas purchased and associated transportation and storage costs during the period.

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No definition available.

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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-Publisher FASB

-Name Accounting Standards Codification

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-Paragraph 3

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- Definition

The increase (decrease) during the reporting period in carrying amount of capitalized gas costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

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-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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