Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, $ in Millions, MMBTU in Trillions	Jun. 30, 2015 USD ($) MMBTU MMBbls	Dec. 31, 2014 USD ($) MMBTU MMBbls
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 391	$ 652
Crude Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	42.0	52.7
Derivative Assets (Liabilities), at Fair Value, Net	$ 195	$ 422
Crude Oil [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	7.4	12.5
Derivative Assets (Liabilities), at Fair Value, Net	$ 196	$ 471
Crude Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	2.2	4.4
Derivative Assets (Liabilities), at Fair Value, Net	$ 21	$ 40
Crude Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	27.9	35.8
Derivative Assets (Liabilities), at Fair Value, Net	$ (26)	$ (89)
Crude Oil [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	4.5	0.0
Derivative Assets (Liabilities), at Fair Value, Net	$ 4	$ 0
Natural Gas [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 63	$ 299
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	604	735
Natural Gas [Member] | Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 135	$ 281
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	260	275
Natural Gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 51	$ 165
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	71	207
Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (128)	$ (170)
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	193	193
Natural Gas [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 5	$ 23
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	80	60
Commodity Contract [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 258	$ 721