Derivative and Hedging Activities - Derivative Instruments Table (Details) MMBbls in Millions, $ in Millions, MMBTU in Trillions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) MMBTU MMBbls	Dec. 31, 2017 MMBTU MMBbls	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (3)		$ 282
Energy related derivative
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ (3)		282
Energy related derivative | Oil
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	37	27
Derivative assets (liabilities), at fair value, net	$ (15)		260
Energy related derivative | Oil | Fixed-price swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	21	12
Derivative assets (liabilities), at fair value, net	$ (31)		157
Energy related derivative | Oil | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	6	8
Derivative assets (liabilities), at fair value, net	$ 24		98
Energy related derivative | Oil | Call swaptions
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	2
Derivative assets (liabilities), at fair value, net	$ (7)
Energy related derivative | Oil | Put options
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	2
Derivative assets (liabilities), at fair value, net	$ (5)
Energy related derivative | Oil | Basis protection swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, volume | MMBbls	6	7
Derivative assets (liabilities), at fair value, net	$ 4		5
Energy related derivative | Natural Gas
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	871	966
Derivative assets (liabilities), at fair value, net	$ 5		15
Energy related derivative | Natural Gas | Fixed-price swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	594	623
Derivative assets (liabilities), at fair value, net	$ 21		26
Energy related derivative | Natural Gas | Three-way collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	66	88
Derivative assets (liabilities), at fair value, net	$ 3		1
Energy related derivative | Natural Gas | Collars
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	28	55
Derivative assets (liabilities), at fair value, net	$ 0		(3)
Energy related derivative | Natural Gas | Call options
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	39	44
Derivative assets (liabilities), at fair value, net	$ 0		0
Energy related derivative | Natural Gas | Call swaptions
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	106	106
Derivative assets (liabilities), at fair value, net	$ (18)		(9)
Energy related derivative | Natural Gas | Basis protection swaps
Derivative [Line Items]
Derivative, nonmonetary notional amount, energy measure | MMBTU	38	50
Derivative assets (liabilities), at fair value, net	$ (1)		0
Future Natural Gas Prices | Energy related derivative | Utica Shale
Derivative [Line Items]
Derivative assets (liabilities), at fair value, net	$ 7		$ 7