Fair Value Measurements (Details) - USD ($)	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$ 0	$ 32,000,000	$ 0
Long-term investment	16,700,000	15,700,000	15,600,000
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2,500,000	0	900,000
Short-term debt	228,000,000	347,000,000	177,000,000
Long-term debt, including current portion	1,244,300,000	1,245,200,000	1,246,100,000
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	32,000,000	0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2,500,000	0	900,000
Short-term debt	228,000,000	347,000,000	177,000,000
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,320,500,000	1,356,100,000	1,367,000,000
Estimated Fair Value [Member] \| Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Long-term investment	16,700,000	15,700,000	15,600,000
Questar Gas [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	19,800,000	0
Notes receivable from Questar	0	0	3,000,000
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	4,100,000	0	4,800,000
Notes payable to Questar	59,700,000	119,300,000	0
Long-term debt, including current portion	534,500,000	534,500,000	534,500,000
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	19,800,000	0
Notes receivable from Questar	0	0	3,000,000
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	4,100,000	0	4,800,000
Notes payable to Questar	59,700,000	119,300,000	0
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	581,500,000	607,200,000	602,000,000
Wexpro [Member] \| Fair Value, Measurements, Nonrecurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Property, Plant, and Equipment, Fair Value Disclosure			0.0
Questar Pipeline [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	1,000,000	7,400,000	6,300,000
Notes receivable from Questar	34,000,000	40,100,000	53,800,000
Financial liabilities [Abstract]
Long-term debt, including current portion	458,700,000	458,800,000	458,800,000
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	1,000,000	7,400,000	6,300,000
Notes receivable from Questar	34,000,000	40,100,000	53,800,000
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 487,600,000	$ 495,500,000	$ 507,700,000

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents affiliated notes payable as of the balance sheet date.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

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-Name Accounting Standards Codification

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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-Name Accounting Standards Codification

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-SubTopic 10

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

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