Employee Benefits Subsidiary plan participation and 401(k) Retirement Income Plan (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Percentage of employees' qualifying earnings to which the employer matching contribution applies (in hundredths)	100.00%
Percentage of employees' qualifying earnings matched by the employer for pre-tax 401(k) Plan purchases (in hundredths)	6.00%
Percentage of employees' qualifying prior year earnings contributed by the employer on a non-matching basis for pre-tax 401(k) Plan purchases (in hundredths)	4.00%
Yearly 401(k) Plan contribution expense amount recognized in the current period	$ 9.1	$ 7.2	$ 7.3
Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	17.6	36.4	38.5
Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	2.9	5.3	6.6
Questar Gas [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	4.1	3.4	3.6
Questar Gas [Member] \| Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	8.5	18.1	19.7
Share of total defined benefit plan contributions	21.8	29.6	39.4
Questar Gas [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	0.8	2.4	3.3
Share of total defined benefit plan contributions	1.1	2.0	1.9
Questar Pipeline [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Yearly 401(k) Plan contribution expense amount recognized in the current period	1.8	1.4	1.4
Questar Pipeline [Member] \| Pension Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	3.2	6.7	7.4
Share of total defined benefit plan contributions	8.1	11.2	15.1
Questar Pipeline [Member] \| Other Postretirement Benefit Plans, Defined Benefit [Member]
Subsidiary plan participation and 401(k) Retirement Income Plan [Line Items]
Net periodic cost	(0.4)	0.2	1.2
Share of total defined benefit plan contributions	$ 0.4	$ 0.7	$ 0.6

Percentage of employee's eligible compensation contributed by the employer to the plan on a non-matching basis.

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (j)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

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Percentage employer matches of the employee's percentage contribution matched.

+ References

No definition available.

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- Definition

Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 80

-Section 50

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450702-114947

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-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450691-114947

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-Publisher FASB

-Name Accounting Standards Codification

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-URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947

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