Asset Quality - Changes in Outstanding Unpaid Principal Balance, Carrying Amount, and Accretable Yield for PCI Loans (Details) - PCI Loans [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accretable Yield
Balance at beginning of period	$ 117	$ 131
Accretion	(10)	(42)
Net reclassifications from nonaccretable to accretable	13	50
Payments received, net	(2)	(21)
Loans charged off		(1)
Balance at end of period	118	117
Carrying Amount	547	571	$ 738
Outstanding Unpaid Principal Balance	$ 578	$ 607	$ 803