CONSOLIDATED STATEMENTS OF EQUITY (USD $) In Millions	Total	Common Stock Including Additional Paid in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Total [Member]	Noncontrolling Interest [Member]	Questar Gas [Member]	Questar Gas [Member] Common Stock [Member]	Questar Gas [Member] Additional Paid-in Capital [Member]	Questar Gas [Member] Retained Earnings [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 Loss [Member]
Beginning Balance at Dec. 31, 2009	$ 3,557.1	$ 454.8	$ 3,077.7	$ (30.3)	$ 3,502.2	$ 54.9	$ 399.0	$ 23.0	$ 148.9	$ 227.1	$ 547.1	$ 6.6	$ 342.7	$ 197.8	$ 0
Beginning 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	(28.8)	0	0	(28.8)	(29.0)	0	0	(29.0)	0
Share-based compensation	22.5	22.5	0	0	22.5	0	1.4	0	1.4	0	1.3	0	1.3	0	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	43.9	0	0	43.9	67.4	0	0	67.4	0
Net other comprehensive income (loss)	(73.7)	0	0	(73.7)	(73.7)	0					0
Ending Balance at Dec. 31, 2010	1,036.1	493.0	647.1	(104.0)	1,036.1	0	415.5	23.0	150.3	242.2	586.8	6.6	344.0	236.2	0
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	(30.3)	0	0	(30.3)	(30.9)	0	0	(30.9)	0
Share-based compensation	9.8	9.8	0	0	9.8	0	1.1	0	1.1	0	1.7	0	1.7	0	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	46.1	0	0	46.1	67.9	0	0	67.9	0
Net other comprehensive income (loss)	(122.1)	0	0	(122.1)	(122.1)	0					(23.4)	0	0	0	(23.4)
Ending Balance at Dec. 31, 2011	1,033.5	514.7	744.9	(226.1)	1,033.5	0	452.4	23.0	171.4	258.0	602.1	6.6	345.7	273.2	(23.4)
Ending Balance (in shares) at Dec. 31, 2011		177.9
Common stock issued	7.2	7.2	0	0	7.2	0
Common stock issued (in shares)		1.5
Common stock repurchased	(88.5)	(88.5)	0	0	(88.5)	0
Common stock repurchased (in shares)		(4.4)
Dividends paid	(117.4)	0	(117.4)	0	(117.4)	0	(33.0)	0	0	(33.0)	(63.0)	0	0	(63.0)	0
Share-based compensation	9.0	9.0	0	0	9.0	0	1.1	0	1.1	0	1.8	0	1.8	0	0
Net income	212.0	0	212.0	0	212.0	0	47.1	0	0	47.1	64.7	0	0	64.7	0
Net other comprehensive income (loss)	(20.2)	0	0	(20.2)	(20.2)	0					0.2	0	0	0	0.2
Ending Balance at Dec. 31, 2012	$ 1,035.6	$ 442.4	$ 839.5	$ (246.3)	$ 1,035.6	$ 0	$ 467.6	$ 23.0	$ 172.5	$ 272.1	$ 605.8	$ 6.6	$ 347.5	$ 274.9	$ (23.2)
Ending Balance (in shares) at Dec. 31, 2012		175.0