Derivative Instruments and Hedging Activities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	$ 167			$ 167			$ 84
Derivative Liability, Fair Value	13			13			10
Realized Mark-to-Market (Gain) Loss	(2)		1	(9)		24
Unrealized Mark-to-Market (Gain) Loss	(159)		(277)	(80)		(204)
Total Gain on Commodity Derivative Instruments	(161)		(276)	(89)		(180)
Deferred Option Premium	12			12
Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	72			72			63
Current Liabilities (1)
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	13	[1]		13	[1]		7
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value	95			95			21
Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value	0			0			3
Crude Oil Commodity Contract
Derivatives, Fair Value [Line Items]
Realized Mark-to-Market (Gain) Loss	6		17	14		51
Unrealized Mark-to-Market (Gain) Loss	(124)		(300)	(83)		(208)
Natural Gas Commodity Contract
Derivatives, Fair Value [Line Items]
Realized Mark-to-Market (Gain) Loss	(8)		(16)	(23)		(27)
Unrealized Mark-to-Market (Gain) Loss	$ (35)		$ 23	$ 3		$ 4

[1] Includes $12 million of deferred put option premium.