Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - E&P [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	$ 520	$ 526	$ 1,157	$ 1,089
Northeast Appalachia [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	213	216	540	469
Southwest Appalachia [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	166	110	322	214
Fayetteville Shale [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	139	198	291	403
Other Property [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	$ 2	$ 2	$ 4	$ 3