Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (16.4)	$ 64.4
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	177.3	(13.5)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	160.9	50.9
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(2.0)	40.8
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	104.3	(27.9)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(14.2)	21.5
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	71.1	15.4
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	(0.2)	2.1
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 1.9	$ (1.0)