Asset Quality - Changes in Outstanding Unpaid Principal Balance, Carrying Amount, and Accretable Yield for PCI Loans (Details) - Purchase Credit Impaired Loans [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accretable Yield
Balance at beginning of period	$ 117	$ 131
Additions	0	0
Accretion	(34)	(42)
Net reclassifications from non-accretable to accretable	25	50
Payments received, net	(14)	(21)
Disposals	0	0
Loans charged off	0	(1)
Balance at end of period	94	117
Carrying Amount	439	571	$ 738
Outstanding Unpaid Principal Balance	$ 462	$ 607	$ 803