Derivative Contracts Gain (Loss) in Statement of Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ 49.5	$ 92.3	$ 113.9	$ 196.7
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(230.0)	(158.3)	(243.5)	(181.8)
Realized and Unrealized Gain (Loss) on Commodity Derivative Contracts Not Designated as Hedging Instruments	(180.5)	(66.0)	(129.6)	14.9
Oil derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	19.9	64.5	60.7	148.5
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(107.7)	(123.7)	(135.6)	(156.8)
Natural gas derivative contracts [Member] | Production [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	28.9	28.1	50.4	46.0
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	(120.2)	(34.5)	(104.8)	(23.1)
Natural gas derivative contracts [Member] | Storage [Member]
Derivative [Line Items]
Realized gain (loss) on commodity derivative contracts not designated as hedging instruments	0.7	(0.3)	2.8	2.2
Unrealized gain (loss) on commodity derivative contracts not designated as hedging instruments	$ (2.1)	$ (0.1)	$ (3.1)	$ (1.9)