Fair Value Measurement (Unobservable inputs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other Assets, Current	$ 560			$ 422
Other Assets, Noncurrent	2,174			2,172
Gulf of Mexico Contingent Consideration [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(5)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at January 1, 2019	127
Net unrealized loss related to assets still held at the end of the period	(5)
Fair value at March 31, 2019	117
Onshore California [Member] | Freeport-McMoRan Oil & Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent Receivable	150
Other Assets, Current	13
Other Assets, Noncurrent	$ 16			$ 16
Onshore California [Member] | Scenario, Forecast [Member] | Freeport-McMoRan Oil & Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent Receivable		$ 50	$ 50