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
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 560	$ 765
Oil, gas and NGL derivatives	720	(605)
Upstream revenues	1,527	314
Total revenues	2,087	1,079
Oil, Gas and NGL Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	807	919
Oil, Gas and NGL Sales [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	662	659
Oil, Gas and NGL Sales [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	70	138
Oil, Gas and NGL Sales [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	75	122
Marketing Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	558	765
Marketing Revenues [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	329	356
Marketing Revenues [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	92	218
Marketing Revenues [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	137	191
Midstream Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	2
Marketing and Midstream Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers	$ 560	$ 765