CONSOLIDATED STATEMENTS OF EQUITY (USD $) In Millions, unless otherwise specified	Total	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Total [Member]	Noncontrolling Interest [Member]	Questar Pipeline [Member]	Questar Pipeline [Member] Common Stock [Member]	Questar Pipeline [Member] Additional Paid-in Capital [Member]	Questar Pipeline [Member] Retained Earnings [Member]	Questar Pipeline [Member] Accumulated Other Comprehensive Income [Member]	Questar Gas [Member]	Questar Gas [Member] Common Stock [Member]	Questar Gas [Member] Additional Paid-in Capital [Member]	Questar Gas [Member] Retained Earnings [Member]
Beginning Balance at Dec. 31, 2008	$ 3,447.5	$ 451.0	$ 2,772.3	$ 194.7	$ 3,418.0	$ 29.5	$ 515.7	$ 6.6	$ 341.6	$ 167.5	$ 0	$ 384.6	$ 23.0	$ 147.9	$ 213.7
Beginning Balance (in shares) at Dec. 31, 2008		173.6
Common stock issued	16.3	16.3	0	0	16.3	0
Common stock issued (in shares)		1.2
Common stock repurchased	(7.2)	(7.2)	0	0	(7.2)	0
Common stock repurchased (in shares)		(0.2)
Dividends paid	(87.9)	0	(87.9)	0	(87.9)	0	(27.9)	0	0	(27.9)	0	(28.2)	0	0	(28.2)
Share-based compensation	22.7	22.7	0	0	22.7	0	1.1	0	1.1	0	0	1.0	0	1.0	0
Tax benefits from share-based compensation	3.6	3.6	0	0	3.6	0
Noncontrolling interest equity adjustment	0	(28.5)	0	0	(28.5)	28.5
Tax on equity adjustment	(3.1)	(3.1)	0	0	(3.1)	0
Distribution to noncontrolling interest	(5.7)	0	0	0	0	(5.7)
Net income	395.9	0	393.3	0	393.3	2.6	58.2	0	0	58.2	0	41.6	0	0	41.6
Other Comprehensive Income (Loss)
Change in unrealized fair value of derivatives	(405.1)	0	0	(405.1)	(405.1)	0
Change in unrecognized actuarial gain	44.5	0	0	44.5	44.5	0
Change in unrecognized prior-service costs	3.3	0	0	3.3	3.3	0
Income taxes	132.3	0	0	132.3	132.3	0
Total comprehensive income	170.9				168.3	2.6
Ending Balance at Dec. 31, 2009	3,557.1	454.8	3,077.7	(30.3)	3,502.2	54.9	547.1	6.6	342.7	197.8	0	399.0	23.0	148.9	227.1
Ending Balance (in shares) at Dec. 31, 2009		174.6
Common stock issued	18.4	18.4	0	0	18.4	0
Common stock issued (in shares)		2.3
Common stock repurchased	(11.1)	(11.1)	0	0	(11.1)	0
Common stock repurchased (in shares)		(0.4)
Dividends paid	(94.8)	0	(94.8)	0	(94.8)	0	(29.0)	0	0	(29.0)	0	(28.8)	0	0	(28.8)
Share-based compensation	22.5	22.5	0	0	22.5	0	1.3	0	1.3	0	0	1.4	0	1.4	0
Tax benefits from share-based compensation	8.4	8.4	0	0	8.4	0
Spinoff of QEP	(2,728.8)	0	(2,675.0)	0	(2,675.0)	(53.8)
Distribution to noncontrolling interest	(2.4)	0	0	0	0	(2.4)
Net income	340.5	0	339.2	0	339.2	1.3	67.4	0	0	67.4	0	43.9	0	0	43.9
Other Comprehensive Income (Loss)
Change in unrealized fair value of derivatives	234.2	0	0	234.2	234.2	0
Unrealized fair value of derivatives transferred to QEP	(372.9)	0	0	(372.9)	(372.9)	0
Change in unrecognized actuarial gain	17.7	0	0	17.7	17.7	0
Change in unrecognized prior-service costs	3.8	0	0	3.8	3.8	0
Change in fair value of long-term investment	0.1	0	0	0.1	0.1	0
Income taxes	43.4	0	0	43.4	43.4	0
Total comprehensive income	266.8				265.5	1.3
Ending Balance at Dec. 31, 2010	1,036.1	493.0	647.1	(104.0)	1,036.1	0	586.8	6.6	344.0	236.2	0	415.5	23.0	150.3	242.2
Ending Balance (in shares) at Dec. 31, 2010		176.5
Common stock issued	7.6	7.6	0	0	7.6	0
Common stock issued (in shares)		1.6
Common stock repurchased	(4.9)	(4.9)	0	0	(4.9)	0
Common stock repurchased (in shares)		(0.2)
Dividends paid	(110.1)	0	(110.1)	0	(110.1)	0	(30.9)	0	0	(30.9)	0	(30.3)	0	0	(30.3)
Share-based compensation	9.8	9.8	0	0	9.8	0	1.7	0	1.7	0	0	1.1	0	1.1	0
Tax benefits from share-based compensation	9.2	9.2	0	0	9.2	0
Equity Contribution from Parent												20.0	0	20.0	0
Net income	207.9	0	207.9	0	207.9	0	67.9	0	0	67.9	0	46.1	0	0	46.1
Other Comprehensive Income (Loss)
Change in unrecognized actuarial gain	(162.9)	0	0	(162.9)	(162.9)	0
Change in unrecognized prior-service costs	3.0	0	0	3.0	3.0	0
Interest rate cash flow hedges	(37.2)	0	0	(37.2)	(37.2)	0	(37.2)	0	0	0	(37.2)
Income taxes	75.0	0	0	75.0	75.0	0	13.8	0	0	0	13.8
Total comprehensive income	85.8				85.8	0	44.5
Ending Balance at Dec. 31, 2011	$ 1,033.5	$ 514.7	$ 744.9	$ (226.1)	$ 1,033.5	$ 0	$ 602.1	$ 6.6	$ 345.7	$ 273.2	$ (23.4)	$ 452.4	$ 23.0	$ 171.4	$ 258.0
Ending Balance (in shares) at Dec. 31, 2011		177.9