Derivatives - Marketing Derivatives and Warrants (Details) shares in Millions	9 Months Ended				12 Months Ended
	Sep. 30, 2020 $ / Barrel	Sep. 30, 2020 $ / MillionCubicFeet	Sep. 30, 2020 MMBbls	Sep. 30, 2020 Bcf	Dec. 31, 2019 $ / Barrel	Dec. 31, 2019 $ / MillionCubicFeet	Dec. 31, 2019 MMBbls	Dec. 31, 2019 Bcf	Jun. 26, 2020 $ / shares shares	May 29, 2020 shares	Aug. 08, 2019 $ / shares shares
Marketing Derivatives | Not designated as hedging instruments
Outstanding commodity derivatives contracts not designated as hedging instruments
Weighted average sales price (dollars per barrel)	41.73	2.36			60.60	2.17
Crude oil | Not designated as hedging instruments | Long position
Outstanding commodity derivatives contracts not designated as hedging instruments
Outstanding net volumes on derivatives not designated as hedges (mmbls/bcf)			5	111			55	128
Common Stock
Outstanding commodity derivatives contracts not designated as hedging instruments
Warrants issued (in shares)									3.9
Exercise price of warrant (usd per share) | $ / shares									$ 59.624
Additional Paid-in Capital | Reclassification
Outstanding commodity derivatives contracts not designated as hedging instruments
Warrants issued (in shares)										103.0
Anadarko | Common Stock
Outstanding commodity derivatives contracts not designated as hedging instruments
Warrants issued (in shares)											80.0
Exercise price of warrant (usd per share) | $ / shares											$ 62.50