Restructuring Charges (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring liability	$ 3	$ 2
Operating lease term				10 years
Restructuring charges	16	11	$ 39
Fayetteville Shale
Restructuring Cost and Reserve [Line Items]
Office consolidation		6	4
Operating lease term				10 years
Restructuring charges	$ 0	11	$ 16
Fayetteville Shale | Office Consolidation and Reorganization
Restructuring Cost and Reserve [Line Items]
Office consolidation		2
Fayetteville Shale | Lease Termination
Restructuring Cost and Reserve [Line Items]
Office consolidation		3
Fayetteville Shale | Other Office Consolidation Expenses
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 1