Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU MMBbls	Dec. 31, 2015 USD ($) MMBTU MMBbls
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (577)	$ 512
Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	28	33
Derivative Assets (Liabilities), at Fair Value, Net	$ (141)	$ 137
Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	23	14
Derivative Assets (Liabilities), at Fair Value, Net	$ (140)	$ 144
Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	5	19
Derivative Assets (Liabilities), at Fair Value, Net	$ (1)	$ (7)
Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	924,000,000	852,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ (363)	$ 130
Natural Gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	719,000,000	500,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ (349)	$ 229
Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	114,000,000	295,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ (99)
Natural Gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	60,000,000	0
Derivative Assets (Liabilities), at Fair Value, Net	$ (9)	$ 0
Natural Gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	31,000,000	57,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ (5)	$ 0
Natural Gas Liquids [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	53	0
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ 0
Commodity Contract [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (504)	$ 267