Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
AFS, Amortized Cost
Due in one year or less	$ 462
Due after one year through five years	544
Due after five years through ten years	2,266
Due after ten years	22,559
Total debt securities	25,831
AFS, Fair Value
Due in one year or less	460
Due after one year through five years	534
Due after five years through ten years	2,161
Due after ten years	21,862
Total debt securities	25,017
HTM, Amortized Cost
Due in one year or less	0
Due after one year through five years	2,699
Due after five years through ten years	863
Due after ten years	18,828
Total debt securities	22,390	$ 23,027
HTM, Fair Value
Due in one year or less	0
Due after one year through five years	2,665
Due after five years through ten years	840
Due after ten years	18,324
Total debt securities	$ 21,829	$ 22,837