Derivative and Hedging Activities - Derivative Instruments Table (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Mar. 31, 2015 MMBbls	Dec. 31, 2014 MMBbls	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 395	$ 652	$ 395	$ 652
Crude Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	46,000,000	52,700,000
Derivative Assets (Liabilities), at Fair Value, Net	365	422	365	422
Crude Oil [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	8,400,000	12,500,000
Derivative Assets (Liabilities), at Fair Value, Net	363	471	363	471
Crude Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	3,300,000	4,400,000
Derivative Assets (Liabilities), at Fair Value, Net	32	40	32	40
Crude Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume	34,300,000	35,800,000
Derivative Assets (Liabilities), at Fair Value, Net	(30)	(89)	(30)	(89)
Natural Gas [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	135	299	135	299
Derivative, Nonmonetary Notional Amount, Energy Measure	550,000,000,000,000	735,000,000,000,000
Natural Gas [Member] | Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	212	281	212	281
Derivative, Nonmonetary Notional Amount, Energy Measure	195,000,000,000,000	275,000,000,000,000
Natural Gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	77	165	77	165
Derivative, Nonmonetary Notional Amount, Energy Measure	106,000,000,000,000	207,000,000,000,000
Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(148)	(170)	(148)	(170)
Derivative, Nonmonetary Notional Amount, Energy Measure	193,000,000,000,000	193,000,000,000,000
Natural Gas [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	(6)	23	(6)	23
Derivative, Nonmonetary Notional Amount, Energy Measure	56,000,000,000,000	60,000,000,000,000
Commodity Contract [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 500	$ 721	$ 500	$ 721