Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (5.9)	$ (43.2)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(181.7)	(53.2)
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(3.0)	(44.3)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(177.3)	(6.9)
Gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(2.9)	0.9
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(0.3)	(2.8)
Gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	0.2
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.0	(0.3)
Total [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(177.6)	(10.0)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	(183.5)	(53.2)
Uinta Basin Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	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	$ 1.8
Pinedale Divestiture [Member]
Derivative [Line Items]
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments		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