Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (16.0)	$ (45.5)	$ (21.9)	$ (88.7)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	38.5	(79.1)	(143.2)	(132.3)
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(16.0)	(52.0)	(19.0)	(96.3)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	54.5	(20.6)	(122.8)	(27.5)
Gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	6.4	(2.9)	7.3
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	(13.0)	(0.3)	(15.8)
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	0.0	0.3
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	0.0	(0.3)
Total [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	54.5	(33.6)	(123.1)	(43.6)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	38.5	(79.1)	(145.0)	(132.3)
Uinta Basin Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0		1.8
Uinta Basin Divestiture [Member] | Gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.0		$ 1.8
Pinedale Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		0.0		0.0
Pinedale Divestiture [Member] | Gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		$ 0.0		$ 0.0