Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, $ in Millions, MMBTU in Trillions		9 Months Ended
	Dec. 31, 2014 USD ($) MMBTU MMBbls	Sep. 30, 2015 USD ($) MMBTU MMBbls
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 652	$ 469
Crude Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	52.7	28.8
Derivative Assets (Liabilities), at Fair Value, Net	$ 422	$ 163
Crude Oil [Member] | Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	12.5	8.0
Derivative Assets (Liabilities), at Fair Value, Net	$ 471	$ 161
Crude Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	4.4	1.1
Derivative Assets (Liabilities), at Fair Value, Net	$ 40	$ 11
Crude Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	35.8	18.1
Derivative Assets (Liabilities), at Fair Value, Net	$ (89)	$ (9)
Crude Oil [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	0.0	1.6
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ 0
Natural Gas [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 299	$ 91
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	735	599
Natural Gas [Member] | Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 281	$ 174
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	275	295
Natural Gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 165	$ 28
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	207	36
Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (170)	$ (107)
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	$ 23	$ (4)
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	60	75
Commodity Contract [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 721	$ 254