Share-Based Compensation Stock options (Detail) (Stock Options [Member], USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 installments	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Stock option minimum term (in years)	5 years
Stock option maximum term (in years)	10 years
Stock option majority minimum term (in years)	7 years
Stock option majority maximum term (in years)	10 years
Minimum number of installments for vesting of options held by employees	3
Maximum number of installments for vesting of options held by employees	4
Number of unvested options at end of period	0		0
Stock option transactions under the terms of the LTSIP - shares [Roll Forward]
Balance at December 31, 2013	717,840
Granted	0
Exercised	(282,333)
Forfeited	0
Balance at December 31, 2014	435,507
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$ 11.84
Exercised	$ 10.88
Balance at December 31, 2014	$ 12.46
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Number of outstanding options at end of period (in shares)		435,507
Weighted-average remaining term (in years and months)	1 year 6 months
Weighted-average exercise price (in dollars per share)	$ 12.46
Number of exercisable options at end of period (in shares)	435,507
Weighted-average exercise price, options exercisable (in dollars per share)	$ 12.46
Stock options, additional disclosures [Abstract]
Aggregate intrinsic value of outstanding options (in shares)	$ 5.6
Aggregate intrinsic value of exercisable options	$ 5.6
Number of Questar stock options held by certain officers, employees and nonemployee directors of QEP (in shares)	245,332
Weighted-average exercise price of Questar stock options awarded to certain officers, employees and nonemployee directors of QEP (in dollars per share)	$ 11.34
Weighted-average remaining life of Questar stock options held by certain officers, employees and nonemployee directors of QEP (in years)	1 year 5 months
Options Outstanding Exercise Price Range 1 [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$ 11.40
Maximum exercise price of options outstanding (in dollars per share)	$ 11.40
Number of outstanding options at end of period (in shares)		239,174
Weighted-average remaining term (in years and months)	1 year 2 months
Weighted-average exercise price (in dollars per share)	$ 11.40
Number of exercisable options at end of period (in shares)	239,174
Weighted-average exercise price, options exercisable (in dollars per share)	$ 11.40
Options Outstanding Exercise Price Range 2 [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$ 13.10
Maximum exercise price of options outstanding (in dollars per share)	$ 17.35
Number of outstanding options at end of period (in shares)		196,333
Weighted-average remaining term (in years and months)	2 years 0 months
Weighted-average exercise price (in dollars per share)	$ 13.75
Number of exercisable options at end of period (in shares)	196,333
Weighted-average exercise price, options exercisable (in dollars per share)	$ 13.75
Minimum [Member]
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$ 7.84
Exercised	$ 7.84
Balance at December 31, 2014	$ 11.40
Maximum [Member]
Stock option transactions under the terms of the LTSIP - exercise price [Roll Forward]
Balance at December 31, 2013	$ 17.35
Exercised	$ 13.24
Balance at December 31, 2014	$ 17.35
Questar Gas [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Number of outstanding options at end of period (in shares)		0
Questar Pipeline [Member]
Stock options outstanding, exercisable and unvested at end of period, by exercise price range [Abstract]
Minimum exercise price of options outstanding (in dollars per share)	$ 11.40
Maximum exercise price of options outstanding (in dollars per share)	$ 13.10
Number of outstanding options at end of period (in shares)		58,933
Weighted-average remaining term (in years and months)	1 year 6 months
Weighted-average exercise price (in dollars per share)	$ 11.95
Number of exercisable options at end of period (in shares)	58,933
Weighted-average exercise price, options exercisable (in dollars per share)	$ 11.95