Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	$ 0.0	$ 32.0	$ 7.3
Long-term investment	14.0	15.7	15.8
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2.8	0.0	0.0
Short-term debt	305.0	347.0	234.0
Long-term debt, including current portion	1,243.8	1,245.2	1,245.7
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	32.0	7.3
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	2.8	0.0	0.0
Short-term debt	305.0	347.0	234.0
Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	1,323.5	1,356.1	1,364.0
Estimated Fair Value [Member] \| Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Long-term investment	14.0	15.7	15.8
Questar Gas [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	19.8	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	3.4	0.0	2.1
Notes payable to Questar	180.0	119.3	90.8
Long-term debt, including current portion	534.5	534.5	534.5
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	0.0	19.8	0.0
Financial liabilities [Abstract]
Checks outstanding in excess of cash balances	3.4	0.0	2.1
Notes payable to Questar	180.0	119.3	90.8
Questar Gas [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	588.7	607.2	602.8
Questar Pipeline [Member] \| Carrying Amount [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.1	7.4	0.4
Notes receivable from Questar	48.2	40.1	35.2
Financial liabilities [Abstract]
Long-term debt, including current portion	458.7	458.8	458.8
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 1 [Member]
Financial assets [Abstract]
Cash and cash equivalents	1.1	7.4	0.4
Notes receivable from Questar	48.2	40.1	35.2
Questar Pipeline [Member] \| Estimated Fair Value [Member] \| Fair Value, Inputs, Level 2 [Member]
Financial liabilities [Abstract]
Long-term debt, including current portion	$ 484.1	$ 495.5	$ 505.0

No definition available.

Name:	str_ChecksOutstandingInExcessOfCashBalancesFairValueDisclosure
Namespace Prefix:	str_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	str_NotesPayableAffiliatedFairValueDisclosure
Namespace Prefix:	str_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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).

+ References

No definition available.

+ Details

Name:	str_NotesReceivableAffiliatedFairValueDisclosure
Namespace Prefix:	str_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph a, f, g

-Article 7

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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 the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_ShorttermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=str_QuestarGasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=str_QuestarPipelineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: