Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (4.9)	$ (38.4)	$ (26.8)	$ (127.1)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	92.3	(63.7)	(30.8)	(107.3)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	87.4	(108.0)	(55.8)	(240.3)
Storage [Member] | Gas derivative contracts [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	0.0	0.3
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.0	0.0	(0.3)
Production [Member] | Oil derivative contracts [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(4.9)	(41.7)	(23.9)	(138.0)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	92.3	(60.6)	(30.5)	(88.1)
Production [Member] | Gas derivative contracts [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	3.3	(2.9)	10.6
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	(3.1)	(0.3)	(18.9)
Total [Member]
Derivative [Line Items]
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	87.4	(102.1)	(57.6)	(234.4)
Uinta Basin [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	(5.9)	1.8	(5.9)
Uinta Basin [Member] | Production [Member] | Oil derivative contracts [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	(2.7)	0.0	(2.7)
Uinta Basin [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	1.8	0.0
Uinta Basin [Member] | Production [Member] | NGL derivative contracts [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.0	$ (3.2)	$ 0.0	$ (3.2)