Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (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 operating revenues	$ 527	$ 636	$ 1,529	$ 2,293
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	308	357	884	1,288
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	298	348	853	1,261
Operating Segments | E&P | Northeast Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	131	175	443	740
Operating Segments | E&P | Southwest Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	166	173	409	519
Operating Segments | E&P | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1	$ 0	$ 1	$ 2