Derivative and Hedging Activities - Derivative Instruments Table (Details) gal in Millions, MMBbls in Millions, $ in Millions, MMBTU in Trillions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) MMBTU gal MMBbls	Dec. 31, 2017 USD ($) MMBTU gal MMBbls
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (318)	$ (35)
Energy related derivative [Member]
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (318)	$ (35)
Energy related derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	40	36
Derivative assets (liabilities), at fair value, net	$ (311)	$ (183)
Energy related derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	26	21
Derivative assets (liabilities), at fair value, net	$ (271)	$ (151)
Energy related derivative [Member] | Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	1	2
Derivative assets (liabilities), at fair value, net	$ (14)	$ (10)
Energy related derivative [Member] | Oil [Member] | Call Swaption [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	2	2
Derivative assets (liabilities), at fair value, net	$ (32)	$ (13)
Energy related derivative [Member] | Oil [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	11	11
Derivative assets (liabilities), at fair value, net	$ 6	$ (9)
Energy related derivative [Member] | Natural gas [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	473	754
Derivative assets (liabilities), at fair value, net	$ 4	$ 150
Energy related derivative [Member] | Natural gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	240	532
Derivative assets (liabilities), at fair value, net	$ 3	$ 149
Energy related derivative [Member] | Natural gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	87	0
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	24	47
Derivative assets (liabilities), at fair value, net	$ 2	$ 11
Energy related derivative [Member] | Natural gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	77	110
Derivative assets (liabilities), at fair value, net	$ 0	$ (3)
Energy related derivative [Member] | Natural gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	45	65
Derivative assets (liabilities), at fair value, net	$ (1)	$ (7)
Energy related derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | gal	114	33
Derivative assets (liabilities), at fair value, net	$ (11)	$ (2)