Derivative and Hedging Activities - Derivative Instruments Table (Details) gal in Millions, MMBbls in Millions, $ in Millions, MMBTU in Trillions	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU MMBbls gal	Dec. 31, 2016 USD ($) MMBTU MMBbls gal
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (35)	$ (577)
Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ (35)	$ (504)
Energy Related Derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	36	28
Derivative Assets (Liabilities), at Fair Value, Net	$ (183)	$ (141)
Energy Related Derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	21	23
Derivative Assets (Liabilities), at Fair Value, Net	$ (151)	$ (140)
Energy Related Derivative [Member] | Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	2	0
Derivative Assets (Liabilities), at Fair Value, Net	$ (10)	$ 0
Energy Related Derivative [Member] | Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	0	5
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ (1)
Energy Related Derivative [Member] | Oil [Member] | Call Swaption [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	2	0
Derivative Assets (Liabilities), at Fair Value, Net	$ (13)	$ 0
Energy Related Derivative [Member] | Oil [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	11	0
Derivative Assets (Liabilities), at Fair Value, Net	$ (9)	$ 0
Energy Related Derivative [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	754	924
Derivative Assets (Liabilities), at Fair Value, Net	$ 150	$ (363)
Energy Related Derivative [Member] | Natural Gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	532	719
Derivative Assets (Liabilities), at Fair Value, Net	$ 149	$ (349)
Energy Related Derivative [Member] | Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	110	114
Derivative Assets (Liabilities), at Fair Value, Net	$ (3)	$ 0
Energy Related Derivative [Member] | Natural Gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	65	31
Derivative Assets (Liabilities), at Fair Value, Net	$ (7)	$ (5)
Energy Related Derivative [Member] | Natural Gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	47	60
Derivative Assets (Liabilities), at Fair Value, Net	$ 11	$ (9)
Energy Related Derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	33	53
Derivative Assets (Liabilities), at Fair Value, Net	$ (2)	$ 0