Derivative and Hedging Activities - Derivative Instruments Table (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Fair Value	$ (726)	$ (551)
Natural Gas [Member]
Derivative [Line Items]
Volume, Energy Measurement	1,125,000,000,000,000	1,009,000,000,000,000
Fair Value	(389)	(237)
Natural Gas [Member] | Swap
Derivative [Line Items]
Volume, Energy Measurement	394,000,000,000,000	448,000,000,000,000
Fair Value	(125)	(23)
Natural Gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Volume, Energy Measurement	387,000,000,000,000	288,000,000,000,000
Fair Value	(48)	(7)
Natural Gas [Member] | Call Option
Derivative [Line Items]
Volume, Energy Measurement	193,000,000,000,000	193,000,000,000,000
Fair Value	(202)	(210)
Natural Gas [Member] | Swaptions
Derivative [Line Items]
Volume, Energy Measurement	0	12,000,000,000,000
Fair Value	0	0
Natural Gas [Member] | Basis Swap [Member]
Derivative [Line Items]
Volume, Energy Measurement	151,000,000,000,000	68,000,000,000,000
Fair Value	(14)	3
Crude Oil [Member]
Derivative [Line Items]
Volume, Volume Measurement	65,000,000	68,200,000
Fair Value	(337)	(314)
Crude Oil [Member] | Swap
Derivative [Line Items]
Volume, Volume Measurement	22,800,000	25,300,000
Fair Value	(81)	(50)
Crude Oil [Member] | Call Option
Derivative [Line Items]
Volume, Volume Measurement	41,900,000	42,500,000
Fair Value	(257)	(265)
Crude Oil [Member] | Basis Swap [Member]
Derivative [Line Items]
Volume, Volume Measurement	300,000	400,000
Fair Value	$ 1	$ 1