Derivative and Hedging Activities - Derivative Instruments Table (Details) gal in Millions, MMBbls in Millions, $ in Millions, MMBTU in Trillions	3 Months Ended
	Mar. 31, 2018 USD ($) MMBTU gal MMBbls	Mar. 31, 2017 MMBTU gal MMBbls	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (152)		$ (35)
Energy Related Derivative [Member]
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (152)		(35)
Energy Related Derivative [Member] | Oil [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	34	36
Derivative assets (liabilities), at fair value, net	$ (206)		(183)
Energy Related Derivative [Member] | Oil [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	24	21
Derivative assets (liabilities), at fair value, net	$ (180)		(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	$ (12)		(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	$ (19)		(13)
Energy Related Derivative [Member] | Oil [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	7	11
Derivative assets (liabilities), at fair value, net	$ 5		(9)
Energy Related Derivative [Member] | Natural gas [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	616	754
Derivative assets (liabilities), at fair value, net	$ 54		150
Energy Related Derivative [Member] | Natural gas [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	358	532
Derivative assets (liabilities), at fair value, net	$ 44		149
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	$ 0		0
Energy Related Derivative [Member] | Natural gas [Member] | Call Option [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	93	110
Derivative assets (liabilities), at fair value, net	$ (1)		(3)
Energy Related Derivative [Member] | Natural gas [Member] | Basis Protection Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	41	65
Derivative assets (liabilities), at fair value, net	$ 3		(7)
Energy Related Derivative [Member] | Natural gas [Member] | Collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	36	47
Derivative assets (liabilities), at fair value, net	$ 8		11
Energy Related Derivative [Member] | NGL [Member] | Fixed-Price Swap [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | gal	47	33
Derivative assets (liabilities), at fair value, net	$ 0		$ (2)