Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (43.2)	$ (16.4)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(10.0)	177.3
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(44.3)	(2.0)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(6.9)	104.3
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.9	(14.2)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(2.8)	71.1
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.2	(0.2)
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(0.3)	1.9
Total [Member]
Derivative [Line Items]
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	$ (53.2)	$ 160.9