Revenue Recognition (Disaggregation of Revenue on Geographic Basis) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 779	$ 527	$ 410	$ 592	$ 745	$ 636	$ 667	$ 990	$ 2,308	$ 3,038	$ 3,862
Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									1,391	1,740	2,551
Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									1,348	1,703	2,525
Northeast Appalachia | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									648	964	1,165
Southwest Appalachia | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									700	736	817
Fayetteville Shale | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	537
Other | Exploration and Production | Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0	$ 3	$ 6