Leases (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended
	Jul. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating lease liability		$ 159	$ 153
Operating lease assets		163	$ 159
Operating lease term	10 years
Payment to previous lessor	$ 6
Operating lease not yet commenced		6
Montage Resources Corporation
Lessee, Lease, Description [Line Items]
Operating lease liability		25
Operating lease assets		$ 25