Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 291.9	$ (35.1)	$ (158.1)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(59.2)	(140.1)	254.4
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	232.7	(175.2)	96.3
Realized and unrealized gains (losses) on derivative contracts	232.7	(173.4)	90.4
Oil derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	296.4	(32.2)	(153.4)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(48.7)	(139.8)	277.0
Gas derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(4.5)	(2.9)	(5.0)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(10.5)	(0.3)	(22.3)
Gas derivative contracts [Member] | Storage [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	0.3
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	(0.3)
Uinta Basin Divestiture [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments			(5.9)
Uinta Basin Divestiture [Member] | Oil derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments			(2.7)
Uinta Basin Divestiture [Member] | Gas derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments			0.0
Uinta Basin Divestiture [Member] | NGL derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments			$ (3.2)
Haynesville Divestiture [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	1.8
Haynesville Divestiture [Member] | Oil derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0
Haynesville Divestiture [Member] | Gas derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	1.8
Haynesville Divestiture [Member] | NGL derivative contracts [Member] | Production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.0	$ 0.0