Derivative Financial Instruments Derivative Financial Instruments Recorded in Balance Sheet(Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Derivative, Fair Value, Net [Abstract]
Derivative Liability Not Designated as Hedging Instrument Fair Value	$ (158)	$ (261)
Derivative Liability, Fair Value, Gross Asset	112	161
Derivative Asset, Fair Value, Gross Liability	(112)	(161)
Derivative Asset Not Designated As Hedging Instrument, Fair Value	166	236
Derivative Asset, Current	52	75
Derivative Asset, Noncurrent	2	0
Derivative Liability, Current	46	97
Derivative Liability, Noncurrent	0	3
Natural gas contracts [Member]
Derivative, Fair Value, Net [Abstract]
Derivative Liability Not Designated as Hedging Instrument Fair Value	(32)	(64)
Derivative Liability, Fair Value, Gross Asset	8	10
Derivative Asset, Fair Value, Gross Liability	(8)	(10)
Derivative Asset Not Designated As Hedging Instrument, Fair Value	10	10
Derivative Asset, Current	0	0
Derivative Asset, Noncurrent	2	0
Derivative Liability, Current	24	51
Derivative Liability, Noncurrent	0	3
Oil contracts [Member]
Derivative, Fair Value, Net [Abstract]
Derivative Liability Not Designated as Hedging Instrument Fair Value	(126)	(197)
Derivative Liability, Fair Value, Gross Asset	104	151
Derivative Asset, Fair Value, Gross Liability	(104)	(151)
Derivative Asset Not Designated As Hedging Instrument, Fair Value	156	226
Derivative Asset, Current	52	75
Derivative Asset, Noncurrent	0	0
Derivative Liability, Current	22	46
Derivative Liability, Noncurrent	$ 0	$ 0