Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
AFS, Amortized Cost
Due in one year or less	$ 468
Due after one year through five years	2,093
Due after five years through ten years	584
Due after ten years	21,718
Total debt securities	24,863	$ 25,009
AFS, Fair Value
Due in one year or less	466
Due after one year through five years	1,982
Due after five years through ten years	573
Due after ten years	20,898
Total debt securities	23,919	24,547
HTM, Amortized Cost
Due in one year or less	1
Due after one year through five years	2,789
Due after five years through ten years	940
Due after ten years	18,019
Total debt securities	21,749	23,027
HTM, Fair Value
Due in one year or less	1
Due after one year through five years	2,739
Due after five years through ten years	909
Due after ten years	17,431
Total debt securities	$ 21,080	$ 22,837