Derivative and Hedging Activities - Derivative Instruments Table (Details) gal in Millions, MMBbls in Millions, $ in Millions, MMBTU in Trillions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) MMBTU gal MMBbls	Dec. 31, 2016 USD ($) MMBTU gal MMBbls
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 9	$ (577)
Energy Related Derivative [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 9	$ (504)
Energy Related Derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	26	28
Derivative Assets (Liabilities), at Fair Value, Net	$ (27)	$ (141)
Energy Related Derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	18	23
Derivative Assets (Liabilities), at Fair Value, Net	$ (17)	$ (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	$ (2)	$ 0
Energy Related Derivative [Member] | Oil [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	1	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	$ (7)	$ 0
Energy Related Derivative [Member] | Oil [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	3	0
Derivative Assets (Liabilities), at Fair Value, Net	$ (1)	$ 0
Energy Related Derivative [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	914	924
Derivative Assets (Liabilities), at Fair Value, Net	$ 38	$ (363)
Energy Related Derivative [Member] | Natural Gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	696	719
Derivative Assets (Liabilities), at Fair Value, Net	$ 38	$ (349)
Energy Related Derivative [Member] | Natural Gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	121	114
Derivative Assets (Liabilities), at Fair Value, Net	$ (7)	$ 0
Energy Related Derivative [Member] | Natural Gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	26	31
Derivative Assets (Liabilities), at Fair Value, Net	$ (1)	$ (5)
Energy Related Derivative [Member] | Natural Gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	71	60
Derivative Assets (Liabilities), at Fair Value, Net	$ 8	$ (9)
Energy Related Derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	15	53
Derivative Assets (Liabilities), at Fair Value, Net	$ (2)	$ 0