Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 990	$ 920
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	551	643
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	542	637
Operating Segments | E&P | Northeast Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	348	327
Operating Segments | E&P | Southwest Appalachia
Disaggregation of Revenue [Line Items]
Total operating revenues	193	156
Operating Segments | E&P | Fayetteville Shale
Disaggregation of Revenue [Line Items]
Total operating revenues	0	152
Operating Segments | E&P | Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1	$ 2