Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 745	$ 636	$ 667	$ 990	$ 1,175	$ 951	$ 816	$ 920	$ 3,038	$ 3,862	$ 3,203
Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									1,740	2,551	2,105
Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating Revenues									1,703	2,525	2,086
Northeast Appalachia | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating Revenues									964	1,165	837
Southwest Appalachia | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating Revenues									736	817	498
Fayetteville Shale | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating Revenues									0	537	743
Other | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating Revenues									$ 3	$ 6	$ 8