Leases (Schedule of Changes in Operating Lease Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Lease Liability [Roll Forward]
Operating Lease, Liability	$ 210	$ 306
Operating Lease Liability Assumed in Exchange for Right-of-Use Asset	33	142
Operating Lease, Contract Modifications Increase (Decrease)	27	4
Gain (Loss) on Disposition of Operating Lease Liabilities	0	(1)
Operating Lease, Liabilities Settled	(163)	(177)
Accretion Expense, Operating Leases	7	13
Present value of lease liabilities	556	572		$ 0
Finance Lease, Liability Assumed in Exchange for Right-of-Use Asset	0	573
Finance Lease, Contract Modifications Increase (Decrease)	0	0
Gain (Loss) on Disposition of Finance Lease Liabilities	0	0
Finance Lease, Liabilities Settled	(33)	(5)
Accretion of finance lease	$ 17	$ 4	$ 0