Summary of Significant Accounting Policies (Schedule of Revenue from Contracts with Customers Disaggregated Based on Type of Good) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 730	$ 1,156	$ 1,495	$ 2,018
Oil, gas and NGL derivatives	140	(487)	(465)	(600)
Upstream revenues	1,191	766	1,654	1,783
Total revenues	1,921	1,922	3,149	3,801
Oil, Gas and NGL Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	1,051	1,253	2,119	2,383
Oil, Gas and NGL Sales [Member] \| Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	753	808	1,414	1,485
Oil, Gas and NGL Sales [Member] \| Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	147	207	380	462
Oil, Gas and NGL Sales [Member] \| NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	151	238	325	436
Marketing Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	730	1,156	1,495	2,018
Marketing Revenues [Member] \| Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	394	766	750	1,297
Marketing Revenues [Member] \| Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	172	160	390	315
Marketing Revenues [Member] \| NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 164	$ 230	$ 355	$ 406

Revenue from the sale of oil, gas and NGL volumes as well as oil, gas and NGL derivative activity.

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Gain (loss) incurred in hedging activities.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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