Summary of Significant Accounting Policies (Schedule of Revenue from Contracts with Customers Disaggregated Based on Type of Good) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 4,673	$ 6,674	$ 8,439
Revenues	4,673	6,674	8,439
Oil, Gas and NGL Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	2,695	3,809	4,085
Revenues	2,695	3,809	4,085
Oil, Gas and NGL Sales [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	2,034	2,988	2,941
Revenues	2,034	2,988	2,941
Oil, Gas and NGL Sales [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	326	391	482
Revenues	326	391	482
Oil, Gas and NGL Sales [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	335	430	662
Revenues	335	430	662
Marketing and Midstream Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	1,978	2,865	4,354
Revenues	1,978	2,865	4,354
Marketing and Midstream Revenues [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	936	1,534	2,745
Revenues	936	1,534	2,745
Marketing and Midstream Revenues [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	488	645	738
Revenues	488	645	738
Marketing and Midstream Revenues [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	554	686	871
Revenues	$ 554	$ 686	$ 871