Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, $ in Millions, MMBTU in Trillions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) MMBTU MMBbls	Dec. 31, 2015 USD ($) MMBTU MMBbls	Mar. 31, 2015 USD ($)
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 515	$ 512	$ 512
Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	36.9	32.7
Derivative Assets (Liabilities), at Fair Value, Net	$ 74	$ 137
Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	21.2	13.5
Derivative Assets (Liabilities), at Fair Value, Net	$ 78	$ 144
Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	15.7	19.2
Derivative Assets (Liabilities), at Fair Value, Net	$ (4)	$ (7)
Natural Gas [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 167	$ 130
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	728	852
Natural Gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 241	$ 229
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	438	500
Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (65)	$ (99)
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	250	295
Natural Gas [Member] | Basis Swap [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (9)	$ 0
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	40	57