Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - E&P [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	$ 633	$ 470	$ 1,790	$ 1,559
Northeast Appalachia [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	254	163	794	632
Southwest Appalachia [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	235	131	557	345
Fayetteville Shale [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	140	175	431	578
Other Property [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	$ 4	$ 1	$ 8	$ 4