Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Abandonment and impairment	$ 0.2	$ 0.0	$ 12.7		$ 2.0
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.4	4.8	0.4	$ 25.0	4.8
Long-term investment	15.1	16.2	15.1	14.1	16.2
Financial liabilities [Abstract]
Short-term debt	458.5	214.0	458.5	457.6	214.0
Long-term debt, including current portion	957.0	1,231.8	957.0	1,206.7	1,231.8
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.4	4.8	0.4	25.0	4.8
Financial liabilities [Abstract]
Short-term debt	458.5	214.0	458.5	457.6	214.0
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,050.4	1,396.3	1,050.4	1,263.7	1,396.3
Estimated Fair Value [Member] \| Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Long-term investment	15.1	16.2	15.1	14.1	16.2
Wexpro [Member]
Fair Value Disclosures [Abstract]
Abandonment and impairment				12.1
Questar Gas [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	0.0	0.0	10.5	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	0.7	0.4	0.7	0.0	0.4
Notes payable to Questar	140.5	35.8	140.5	273.3	35.8
Long-term debt, including current portion	527.6	526.8	527.6	527.4	526.8
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	10.5	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	0.7	0.4	0.7	0.0	0.4
Notes payable to Questar	140.5	35.8	140.5	273.3	35.8
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	591.7	629.0	591.7	568.4	629.0
Questar Pipeline [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.2	6.1	1.2	10.2	6.1
Notes receivable from Questar	33.2	35.9	33.2	6.0	35.9
Financial liabilities [Abstract]
Long-term debt, including current portion	429.4	453.9	429.4	429.2	453.9
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.2	6.1	1.2	10.2	6.1
Notes receivable from Questar	33.2	35.9	33.2	6.0	35.9
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 458.7	$ 515.1	$ 458.7	$ 445.2	$ 515.1

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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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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. An amount representing an agreement for an unconditional promise by the maker to pay the affiliated Company (holder) a definite sum of money at a future date(s).

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