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, 2018 USD ($) MMBTU gal MMBbls	Dec. 31, 2017 USD ($) MMBTU gal MMBbls
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (338)	$ (35)
Energy related derivative [Member]
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (338)	$ (35)
Energy related derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	29	36
Derivative assets (liabilities), at fair value, net	$ (300)	$ (183)
Energy related derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	21	21
Derivative assets (liabilities), at fair value, net	$ (294)	$ (151)
Energy related derivative [Member] | Oil [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	0	2
Derivative assets (liabilities), at fair value, net	$ (8)	$ (10)
Energy related derivative [Member] | Oil [Member] | Call Swaption [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	0	2
Derivative assets (liabilities), at fair value, net	$ 0	$ (13)
Energy related derivative [Member] | Oil [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	8	11
Derivative assets (liabilities), at fair value, net	$ 2	$ (9)
Energy related derivative [Member] | Natural gas [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	658	754
Derivative assets (liabilities), at fair value, net	$ (23)	$ 150
Energy related derivative [Member] | Natural gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	400	532
Derivative assets (liabilities), at fair value, net	$ (19)	$ 149
Energy related derivative [Member] | Natural gas [Member] | Fixed-Price Swap [Member] | Disposal group, disposed of by sale, not discontinued operations [Member] | Utica Shale [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	170
Energy related derivative [Member] | Natural gas [Member] | Three Way Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	88	0
Derivative assets (liabilities), at fair value, net	$ 3	$ 0
Energy related derivative [Member] | Natural gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	66	47
Derivative assets (liabilities), at fair value, net	$ 0	$ 11
Energy related derivative [Member] | Natural gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	60	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	44	65
Derivative assets (liabilities), at fair value, net	$ (7)	$ (7)
Energy related derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | gal	57	33
Derivative assets (liabilities), at fair value, net	$ (15)	$ (2)