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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2018	Jul. 31, 2016	Jan. 31, 2015
Debt Instrument [Line Items]
Stated interest rate		7.75%
Impairments		$ 16	$ 171	$ 0
Nonrecurring
Debt Instrument [Line Items]
Impairments			161
Other non-core assets
Debt Instrument [Line Items]
Impairments	$ 1	16	11
Other non-core assets \| Nonrecurring
Debt Instrument [Line Items]
Impairments		16	1
Carrying value of non core assets		$ 26
Marketing \| Nonrecurring
Debt Instrument [Line Items]
Impairments			145
Exploration and Production \| Nonrecurring
Debt Instrument [Line Items]
Impairments			$ 15
Senior Notes \| 4.05% Senior Notes Due January 2020
Debt Instrument [Line Items]
Stated interest rate		4.05%	4.05%		5.30%	5.80%	4.05%
Senior Notes \| 4.10% Senior Notes Due January March 2022
Debt Instrument [Line Items]
Stated interest rate		4.10%