Derivative Instruments and Hedging Activities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 46			$ 46			$ 84
Derivative Liability, Fair Value	41			41			10
Realized Mark-to-Market (Gain) Loss	10		4	2		28
Unrealized Mark-to-Market (Gain) Loss	147		131	67		(74)
Total Gain on Commodity Derivative Instruments	157		135	69		(46)
Deferred Option Premium	6			6
Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	15			15			63
Current Liabilities (1)
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	39	[1]		39	[1]		7
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	31			31			21
Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	2			2			3
Crude Oil Commodity Contract
Derivatives, Fair Value [Line Items]
Realized Mark-to-Market (Gain) Loss	24		17	39		68
Unrealized Mark-to-Market (Gain) Loss	143		112	60		(97)
Natural Gas Commodity Contract
Derivatives, Fair Value [Line Items]
Realized Mark-to-Market (Gain) Loss	(14)		(13)	(37)		(40)
Unrealized Mark-to-Market (Gain) Loss	$ 4		$ 19	$ 7		$ 23

[1] Includes $6 million of deferred put option premium.