Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 69.6	$ (4.9)	$ 232.6		$ (26.8)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(103.8)	92.3	84.4		(30.8)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	(34.2)	87.4	317.0		(57.6)
Gain (Loss) on Derivative Instruments, Net, Pretax	(34.2)	87.4	317.0		(55.8)
Production [Member] | Oil derivative contracts [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	70.3	(4.9)	233.1		(23.9)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(91.7)	92.3	105.0		(30.5)
Production [Member] | Gas derivative contracts [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(0.7)	0.0	(0.5)		(2.9)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(12.1)	0.0	(20.6)		(0.3)
Haynesville Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	0.0	[1]	1.8	[1]
Haynesville Divestiture [Member] | Production [Member] | Gas derivative contracts [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.0	$ 0.0	$ 0.0		$ 1.8

[1] During the nine months ended September 30, 2019, the unrealized gains (losses) on commodity derivative contracts related to the Haynesville Divestiture were comprised of derivatives included as part of the Haynesville/Cotton Valley purchase and sale agreement, which were subsequently novated to the buyer upon the closing of the sale in January 2019. Refer to Note 3 – Acquisitions and Divestitures for more information. The unrealized gains (losses) on commodity derivatives associated with the Haynesville Divestiture are offset by an equal amount recorded within "Net gain (loss) from asset sales, inclusive of restructuring costs" on the statements of operations.