Summary of Significant Accounting Policies (Schedule of Revenue from Contracts with Customers Disaggregated Based on Type of Good or Service) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 1,154	$ 1,619	$ 3,276	$ 4,969
Oil, Gas and NGL Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	678	919	1,909	2,774
Oil, Gas and NGL Sales [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	504	751	1,462	2,160
Oil, Gas and NGL Sales [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	79	80	221	291
Oil, Gas and NGL Sales [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	95	88	226	323
Marketing Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	473	698	1,360	2,193
Marketing Revenues [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	233	407	702	1,157
Marketing Revenues [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	97	140	268	530
Marketing Revenues [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	143	151	390	506
Midstream Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	3	2	7	2
Marketing and Midstream Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 476	$ 700	$ 1,367	$ 2,195