Derivative Financial Instruments (Textuals) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Natural gas contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Total	$ 103
Oil contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Total	510
Year2015 [Member] | Natural gas contracts [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price	4.28
Derivative Asset, Fair Value, Total	62
Volume In Mmbtus	49,275,000
Year2015 [Member] | Natural gas contracts [Member] | Collars [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price	3.93
Derivative, Average Cap Price	4.74
Derivative Asset, Fair Value, Total	36
Volume In Mmbtus	38,325,000
Year2015 [Member] | Oil contracts [Member] | Swap [Member]
Derivative [Line Items]
Volume in MBbls	2,275,000
Derivative, Swap Type, Average Fixed Price	90.20
Derivative Asset, Fair Value, Total	78
Year2015 [Member] | Oil contracts [Member] | Fixed Price Swaps With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	15,019,000
Year2015 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price	90.02
Derivative Asset, Fair Value, Total	495
Year2015 [Member] | Oil contracts [Member] | Collars With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	730,000
Year2015 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	71.67
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(258)
Year2015 [Member] | Oil contracts [Member] | Collars with Sold Puts - Collar [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price	90
Derivative, Average Cap Price	104
Derivative Asset, Fair Value, Total	25
Year2015 [Member] | Oil contracts [Member] | Collars with Sold Puts - Sold Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	75
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(15)
Year 2016 [Member] | Natural gas contracts [Member] | Swaption [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price	4.10
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(2)
Volume In Mmbtus	14,640,000
Year 2016 [Member] | Natural gas contracts [Member] | Collars [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price	4
Derivative, Average Cap Price	4.54
Derivative Asset, Fair Value, Total	7
Volume In Mmbtus	10,980,000
Year 2016 [Member] | Oil contracts [Member] | Swaption [Member]
Derivative [Line Items]
Volume in MBbls	732,000
Derivative, Swap Type, Average Fixed Price	91.00
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	0
Year 2016 [Member] | Oil contracts [Member] | Fixed Price Swaps With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	10,060,000
Year 2016 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price	89.98
Derivative Asset, Fair Value, Total	266
Year 2016 [Member] | Oil contracts [Member] | Collars With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	6,220,000
Year 2016 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	74.14
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(174)
Year 2016 [Member] | Oil contracts [Member] | Collars with Sold Puts - Collar [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price	90
Derivative, Average Cap Price	96.15
Derivative Asset, Fair Value, Total	170
Year 2016 [Member] | Oil contracts [Member] | Collars with Sold Puts - Sold Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	75
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(112)
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	4,468,000
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Swap [Member]
Derivative [Line Items]
Derivative, Swap Type, Average Fixed Price	88.37
Derivative Asset, Fair Value, Total	93
Year2017 [Member] | Oil contracts [Member] | Collars With Sold Puts [Member]
Derivative [Line Items]
Volume in MBbls	2,080,000
Year2017 [Member] | Oil contracts [Member] | Fixed Price Swaps with Sold Puts - Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	73.28
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	(72)
Year2017 [Member] | Oil contracts [Member] | Collars with Sold Puts - Collar [Member]
Derivative [Line Items]
Derivative, Weighted Average Floor Price	90
Derivative, Average Cap Price	95.59
Derivative Asset, Fair Value, Total	50
Year2017 [Member] | Oil contracts [Member] | Collars with Sold Puts - Sold Put [Member]
Derivative [Line Items]
Derivative Average Additional Put Price	75
Derivative Instruments Not Designated as Hedging Instruments, Liability, at Fair Value	$ (36)