Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, MMBTU in Millions, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) MMBTU MMBbls	Dec. 31, 2016 USD ($) MMBTU MMBbls
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 81	$ (577)
Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 81	$ (504)
Energy Related Derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	16	28
Derivative Assets (Liabilities), at Fair Value, Net	$ 45	$ (141)
Energy Related Derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	13	23
Derivative Assets (Liabilities), at Fair Value, Net	$ 45	$ (140)
Energy Related Derivative [Member] | Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	3	5
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ (1)
Energy Related Derivative [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	878,000,000	924,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ 36	$ (363)
Energy Related Derivative [Member] | Natural Gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	704,000,000	719,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ 28	$ (349)
Energy Related Derivative [Member] | Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	90,000,000	114,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ 0
Energy Related Derivative [Member] | Natural Gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	71,000,000	60,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ 10	$ (9)
Energy Related Derivative [Member] | Natural Gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	13,000,000	31,000,000
Derivative Assets (Liabilities), at Fair Value, Net	$ (2)	$ (5)
Energy Related Derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	0	53
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ 0