Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - E&P [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 637	$ 563
Northeast Appalachia [Member]
Disaggregation of Revenue [Line Items]
Revenues	327	253
Southwest Appalachia [Member]
Disaggregation of Revenue [Line Items]
Revenues	156	104
Fayetteville Shale [Member]
Disaggregation of Revenue [Line Items]
Revenues	152	205
Other Property [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 2	$ 1