Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 19.6	$ 119.8	$ 133.5	$ 316.5
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	24.9	33.8	(218.6)	(148.0)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	44.5	153.6	(85.1)	168.5
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	19.1	96.8	79.8	245.3
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(0.3)	28.8	(135.9)	(128.0)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.4	23.1	50.8	69.1
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	24.8	3.6	(80.0)	(19.5)
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.1	(0.1)	2.9	2.1
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 0.4	$ 1.4	$ (2.7)	$ (0.5)