Derivative and Hedging Activities Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Fair Value	$ (425)	$ (924)
Natural Gas Trades
Derivative [Line Items]
Volume, Energy Measurement	869,000,000,000,000	353,000,000,000,000
Fair Value	(138)	(231)
Natural Gas Trades | Swap
Derivative [Line Items]
Volume, Energy Measurement	519,000,000,000,000	49,000,000,000,000
Fair Value	94	24
Natural Gas Trades | Call Option
Derivative [Line Items]
Volume, Energy Measurement	193,000,000,000,000	193,000,000,000,000
Fair Value	(227)	(240)
Natural Gas Trades | Swaptions
Derivative [Line Items]
Volume, Energy Measurement	12,000,000,000,000	0
Fair Value	(1)	0
Natural Gas Trades | Basis Protection Swaps
Derivative [Line Items]
Volume, Energy Measurement	90,000,000,000,000	111,000,000,000,000
Fair Value	(7)	(15)
Natural Gas Trades | Three Way Collar [Member]
Derivative [Line Items]
Volume, Energy Measurement	55,000,000,000,000	0
Fair Value	3	0
Crude Oil
Derivative [Line Items]
Volume, Volume Measurement	110,400,000	112,700,000
Fair Value	(287)	(693)
Crude Oil | Swap
Derivative [Line Items]
Volume, Volume Measurement	36,800,000	28,100,000
Fair Value	75	68
Crude Oil | Call Option
Derivative [Line Items]
Volume, Volume Measurement	67,600,000	73,800,000
Fair Value	(363)	(748)
Crude Oil | Swaptions
Derivative [Line Items]
Volume, Volume Measurement	5,300,000	5,300,000
Fair Value	(2)	(13)
Crude Oil | Basis Protection Swaps
Derivative [Line Items]
Volume, Volume Measurement	700,000	5,500,000
Fair Value	$ 3	$ 0