Discontinued Operations (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discontinued Operations and Disposal Groups [Abstract]
Revenues	$ 0	$ 0	$ 1,109.8
Operating income	0	0	270.5
Discontinued operations, net of income taxes	0	0	148.2
Discontinued operations, noncontrolling interest	0	0	(1.3)
Total Discontinued Operations, Net Of Income Taxes	0	0	146.9
Earnings Per Common Share Attributable To Questar
Basic from discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.84
Diluted from discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.83
Discontinued operations income taxes	$ 0	$ 0	$ 86.3