Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
AFS, Amortized Cost
Due in one year or less	$ 425
Due after one year through five years	2,660
Due after five years through ten years	1,038
Due after ten years	21,566
Total debt securities	25,689	$ 25,009
AFS, Fair Value
Due in one year or less	424
Due after one year through five years	2,587
Due after five years through ten years	1,043
Due after ten years	20,984
Total debt securities	25,038	24,547
HTM, Amortized Cost
Due in one year or less	1
Due after one year through five years	3,290
Due after five years through ten years	627
Due after ten years	16,634
Total debt securities	20,552	23,027
HTM, Fair Value
Due in one year or less	1
Due after one year through five years	3,244
Due after five years through ten years	611
Due after ten years	16,191
Total debt securities	$ 20,047	$ 22,837