Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 667	$ 816	$ 1,657	$ 1,736
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	380	527	931	1,170
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	371	520	913	1,157
Operating Segments | E&P | Northeast Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	217	213	565	540
Operating Segments | E&P | Southwest Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	153	166	346	322
Operating Segments | E&P | Fayetteville Shale
Disaggregation of Revenue [Line Items]
Total operating revenues	0	139	0	291
Operating Segments | E&P | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1	$ 2	$ 2	$ 4