Leases (Summary of Lease Expense Components) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Total lease cost	$ 723	$ 802	$ 1,693	$ 1,876
PPA leases
Lessee, Lease, Description [Line Items]
Operating lease cost	35	38	74	98
Finance lease cost	1	1	2	1
Variable lease cost	676	751	1,582	1,742
Total lease cost	712	790	1,658	1,841
Other operating leases cost
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 11	$ 12	$ 35	$ 35