Derivative and Hedging Activities Derivative Instruments Table (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Fair Value	$ (583)	$ (924)
Natural Gas [Member]
Derivative [Line Items]
Volume, Energy Measurement	661,000,000,000,000	353,000,000,000,000
Fair Value	(121)	(231)
Natural Gas [Member] | Swap
Derivative [Line Items]
Volume, Energy Measurement	341,000,000,000,000	49,000,000,000,000
Fair Value	96	24
Natural Gas [Member] | Call Option
Derivative [Line Items]
Volume, Energy Measurement	193,000,000,000,000	193,000,000,000,000
Fair Value	(219)	(240)
Natural Gas [Member] | Swaptions
Derivative [Line Items]
Volume, Energy Measurement	12,000,000,000,000	0
Fair Value	0	0
Natural Gas [Member] | Basis Swap [Member]
Derivative [Line Items]
Volume, Energy Measurement	79,000,000,000,000	111,000,000,000,000
Fair Value	(2)	(15)
Natural Gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Volume, Energy Measurement	36,000,000,000,000	0
Fair Value	4	0
Crude Oil [Member]
Derivative [Line Items]
Volume, Volume Measurement	103,000,000	112,700,000
Fair Value	(462)	(693)
Crude Oil [Member] | Swap
Derivative [Line Items]
Volume, Volume Measurement	31,600,000	28,100,000
Fair Value	(94)	68
Crude Oil [Member] | Call Option
Derivative [Line Items]
Volume, Volume Measurement	65,600,000	73,800,000
Fair Value	(368)	(748)
Crude Oil [Member] | Swaptions
Derivative [Line Items]
Volume, Volume Measurement	5,300,000	5,300,000
Fair Value	(1)	(13)
Crude Oil [Member] | Basis Swap [Member]
Derivative [Line Items]
Volume, Volume Measurement	500,000	5,500,000
Fair Value	$ 1	$ 0