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, 2018	Dec. 31, 2017	Dec. 31, 2016
Accretable Yield
Balance at beginning of period	$ 131	$ 197
Additions	0	(32)
Accretion	(42)	(44)
Net reclassifications from non-accretable to accretable	50	15
Payments received, net	(21)	(4)
Disposals	0	(1)
Loans charged off	(1)	0
Balance at end of period	117	131
Carrying Amount	571	738	$ 865
Outstanding Unpaid Principal Balance	$ 607	$ 803	$ 1,002