Derivatives - Not Designated as Hedging Instruments (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) $ / MillionCubicFeet $ / Barrel Bcf MMBbls	Dec. 31, 2018 USD ($) $ / MillionCubicFeet $ / Barrel Bcf MMBbls
Outstanding commodity derivatives contracts not designated as hedging instruments
Derivative instrument settlement period (within)	3 months
Not designated as hedging instruments | Crude oil
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price (dollars per barrel) | $ / Barrel	61.46	58.81
Not designated as hedging instruments | Crude oil | Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges	$ (24)	$ 184
Not designated as hedging instruments | Natural gas
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price (dollars per barrel) | $ / MillionCubicFeet	2.11	3.18
Not designated as hedging instruments | Natural gas | Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges	$ 5	$ 5
Not designated as hedging instruments | Long position | Crude oil
Outstanding commodity derivatives contracts not designated as hedging instruments
Outstanding net volumes on derivatives not designated as hedges (mmbls/bcf) | MMBbls	54	61
Not designated as hedging instruments | Short position | Natural gas
Outstanding commodity derivatives contracts not designated as hedging instruments
Outstanding net volumes on derivatives not designated as hedges (mmbls/bcf) | Bcf	155	142