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, 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				267.2		3.2
Dividends	(14.1)				(14.1)
Equity-based compensation	24.1		(9.2)	33.3
Stock Issued During Period, Shares, Equity-based Compensation, Net of Forfeitures		1.3
Stock Issued During Period, Shares, Treasury Stock Reissued			(0.3)
Proceeds from Issuance Initial Public Offering	0
Payments to Noncontrolling Interests	(5.4)						(5.4)
Equity from Questar	0.4				0.4
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	24.8					24.8
Total pension and other postretirement plans adjustments	(11.2)					(11.2)
Treasury stock (in shares)			(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				128.3		3.7
Dividends	(14.2)				(14.2)
Equity-based compensation	23.2		7.1	30.7	(14.6)
Stock Issued During Period, Shares, Equity-based Compensation, Net of Forfeitures		1.3
Stock Issued During Period, Shares, Treasury Stock Reissued			0.2
Distribution to QEP Education Foundation	2.3		2.3
Proceeds from Issuance Initial Public Offering	0
Payments to Noncontrolling Interests	(6.6)						6.6
TreasuryStockIssuedCharitableContribution			0.1
Gain reclassified from AOCI into income for effective portion of hedge	(171.1)					(171.1)
Total pension and other postretirement plans adjustments	(4.0)					(4.0)
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
NET INCOME	171.4				159.4		12.0
Dividends	(14.3)				(14.3)
Equity-based compensation	25.0		(11.2)	36.3	(0.3)		0.2
Stock Issued During Period, Shares, Equity-based Compensation, Net of Forfeitures		0.8
Stock Issued During Period, Shares, Treasury Stock Reissued			(0.3)
Proceeds from Issuance Initial Public Offering	449.6						449.6
Payments to Noncontrolling Interests	(9.3)						(9.3)
Gain reclassified from AOCI into income for effective portion of hedge	(77.6)					(77.6)
Total pension and other postretirement plans adjustments	18.3					18.3
Treasury stock (in shares)	0.4		(0.4)
Balance at Dec. 31, 2013	$ 3,876.8	$ 1.8	$ (14.9)	$ 498.4	$ 2,917.8	$ (26.5)	$ 500.2
Shares, Issued at Dec. 31, 2013		179.3