Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment	$ 0.1	$ 0.1	$ 0.3	$ 0.1	$ 12.7		$ 0.1
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment			0.0	0.0
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	$ 25.0	0.0
Long-term investment	16.6	16.7	16.6	16.7	16.6	14.1	16.7
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	4.0	2.5	4.0	2.5	4.0	0.0	2.5
Short-term debt	459.0	228.0	459.0	228.0	459.0	457.6	228.0
Long-term debt, including current portion	962.6	1,237.8	962.6	1,237.8	962.6	1,212.3	1,237.8
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	25.0	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	4.0	2.5	4.0	2.5	4.0	0.0	2.5
Short-term debt	459.0	228.0	459.0	228.0	459.0	457.6	228.0
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,086.5	1,320.5	1,086.5	1,320.5	1,086.5	1,263.7	1,320.5
Estimated Fair Value [Member] \| Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Long-term investment	16.6	16.7	16.6	16.7	16.6	14.1	16.7
Wexpro [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Abandonment and impairment						12.1
Questar Gas [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	10.5	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	5.1	4.1	5.1	4.1	5.1	0.0	4.1
Notes payable to Questar	129.0	59.7	129.0	59.7	129.0	273.3	59.7
Long-term debt, including current portion	531.4	531.0	531.4	531.0	531.4	531.2	531.0
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	0.0	0.0	0.0	0.0	10.5	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	5.1	4.1	5.1	4.1	5.1	0.0	4.1
Notes payable to Questar	129.0	59.7	129.0	59.7	129.0	273.3	59.7
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	618.5	581.5	618.5	581.5	618.5	568.4	581.5
Questar Pipeline [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.6	1.0	1.6	1.0	1.6	10.2	1.0
Notes receivable from Questar	34.6	34.0	34.6	34.0	34.6	6.0	34.0
Financial liabilities [Abstract]
Long-term debt, including current portion	431.2	456.0	431.2	456.0	431.2	431.0	456.0
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.6	1.0	1.6	1.0	1.6	10.2	1.0
Notes receivable from Questar	34.6	34.0	34.6	34.0	34.6	6.0	34.0
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 468.0	$ 487.6	$ 468.0	$ 487.6	$ 468.0	$ 445.2	$ 487.6

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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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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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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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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
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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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