Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 410	$ 667	$ 1,002	$ 1,657
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	223	380	576	931
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	211	371	555	913
Operating Segments | E&P | Northeast Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	117	217	312	565
Operating Segments | E&P | Southwest Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	94	153	243	346
Operating Segments | E&P | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0	$ 1	$ 0	$ 2