CONSOLIDATED STATEMENTS OF EQUITY (USD $) In Millions, unless otherwise specified	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest [Member]
Balance at Dec. 31, 2009	$ 2,808.7	$ 1.7	$ 0	$ 126.8	$ 2,538.2	$ 87.1	$ 54.9
Shares, Issued at Dec. 31, 2009		174.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Questar common stock issued net of repurchases, Value	0	0	0	0	0	0	0
Questar common stock issued, net of repurchases		0.4
NET INCOME	329.1	0	0	0	326.2	0	2.9
Payments of Ordinary Dividends, Common Stock	(7.0)
Dividends	(15.9)	0	0	0	(15.9)	0	0
Share-based compensation	19.5	0.1	(3.9)	23.3	0	0	0
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures		0.9
Stock Issued During Period, Shares, Treasury Stock Reissued			(0.1)
Stockholders' Equity Note, Spinoff Transaction	250.0	0	0	250.0	0	0	0
Payments to Noncontrolling Interests	(5.0)	0	0	0	0	0	(5.0)
Transfer Wexpro to Questar	(430.5)	0	0	(2.0)	(428.5)	0	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	136.7	0	0	0	0	136.7	0
Change in pension and postretirement liability, net of tax	(29.5)	0	0	0	0	(29.5)	0
Treasury stock (in shares)			(0.1)
Balance at Dec. 31, 2010	3,063.1	1.8	(3.9)	398.1	2,420.0	194.3	52.8
Shares, Issued at Dec. 31, 2010		175.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
NET INCOME	270.4	0	0	0	267.2	0	3.2
Payments of Ordinary Dividends, Common Stock	(14.1)
Dividends	(14.1)	0	0	0	(14.1)	0	0
Share-based compensation	24.1	0	(9.2)	33.3	0	0	0
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures		1.3
Stock Issued During Period, Shares, Treasury Stock Reissued			(0.3)
Payments to Noncontrolling Interests	(5.4)	0	0	0	0	0	(5.4)
Equity from Questar	0.4	0	0	0	0.4	0	0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	24.8	0	0	0	0	24.8	0
Change in pension and postretirement liability, net of tax	(11.2)	0	0	0	0	(11.2)	0
Treasury stock (in shares)	0.4		(0.4)
Balance at Dec. 31, 2011	3,352.1	1.8	(13.1)	431.4	2,673.5	207.9	50.6
Shares, Issued at Dec. 31, 2011		177.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
NET INCOME	132.0	0	0	0	128.3	0	3.7
Payments of Ordinary Dividends, Common Stock	(14.2)
Dividends	(14.2)	0	0	0	(14.2)	0	0
Share-based compensation	23.2	0	7.1	30.7	(14.6)	0	0
Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures		1.3
Stock Issued During Period, Shares, Treasury Stock Reissued			0.2
Distribution to QEP Education Foundation	2.3	0	2.3	0	0	0	0
Payments to Noncontrolling Interests	(6.6)	0	0	0	0	0	(6.6)
TreasuryStockIssuedCharitableContribution			0.1
Gain reclassified from AOCI into income for effective portion of hedge	(171.1)	0	0	0	0	(171.1)	0
Change in pension and postretirement liability, net of tax	(4.0)	0	0	0	0	(4.0)	0
Treasury stock (in shares)	0.1		(0.1)
Balance at Dec. 31, 2012	$ 3,313.7	$ 1.8	$ (3.7)	$ 462.1	$ 2,773.0	$ 32.8	$ 47.7
Shares, Issued at Dec. 31, 2012		178.5