Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 636	$ 951	$ 2,293	$ 2,687
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	357	639	1,288	1,809
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	348	633	1,261	1,790
Operating Segments | E&P | Northeast Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	175	254	740	794
Operating Segments | E&P | Southwest Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	173	235	519	557
Operating Segments | E&P | Fayetteville Shale
Disaggregation of Revenue [Line Items]
Total operating revenues	0	140	0	431
Operating Segments | E&P | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0	$ 4	$ 2	$ 8