Derivatives - Not Designated as Hedging Instruments (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / bbl MMBbls Bcf	Dec. 31, 2017 USD ($) $ / bbl MMBbls Bcf
Crude Oil (in barrels) | Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges | $	$ (73)	$ (47)
Natural gas (in cubic feet) | Net sales
Outstanding commodity derivatives contracts not designated as hedging instruments
Unrealized gain (loss) on derivatives not designated as hedges | $		$ 1
Long position | Crude Oil (in barrels)
Outstanding commodity derivatives contracts not designated as hedging instruments
Derivative, Nonmonetary Notional Amount, Volume | MMBbls	45	61
Short position | Natural gas (in cubic feet)
Outstanding commodity derivatives contracts not designated as hedging instruments
Derivative, Nonmonetary Notional Amount, Volume | Bcf	57	47
Not designated as hedging instruments | Crude Oil (in barrels)
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price | $ / bbl	70.90	57.38
Not designated as hedging instruments | Natural gas (in cubic feet)
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price | $ / bbl	2.53	2.73