Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments		$ 11.7	$ 19.6	$ 1.7	$ 133.5
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		(86.1)	24.9	191.5	(218.6)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments		(104.3)	44.5	163.3	(85.1)
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments		12.1	19.1	21.6	79.8
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		(86.1)	(0.3)	88.7	(135.9)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments		(0.4)	0.4	(19.7)	50.8
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		0.0	24.8	100.5	(80.0)
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments		0.0	0.1	(0.2)	2.9
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		0.0	0.4	2.3	(2.7)
Pinedale Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	[1]	(29.9)	0.0	(29.9)	0.0
Pinedale Divestiture [Member] | NGL derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	[1]	(5.1)	0.0	(5.1)	0.0
Pinedale Divestiture [Member] | Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	[1]	(1.3)	0.0	(1.3)	0.0
Pinedale Divestiture [Member] | Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	[1]	(23.5)	0.0	(23.5)	0.0
Total [Member]
Derivative [Line Items]
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments		$ (74.4)	$ 44.5	$ 193.2	$ (85.1)

[1] The unrealized gains (losses) on commodity derivative contracts related to the Pinedale Divestiture are comprised of derivatives entered into in conjunction with the execution of the Pinedale purchase and sale agreement, which were subsequently novated to the buyer upon the closing of the sale in September 2017. Refer to Note 2 – Acquisitions and Divestitures for more information. The unrealized gains (losses) on commodity derivatives associated with the Pinedale Divestiture are offset by an equal amount recorded within "Net gain (loss) from asset sales" on the Condensed Consolidated Statements of Operations.