Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
AFS, Amortized Cost
Due in one year or less	$ 481
Due after one year through five years	2,114
Due after five years through ten years	561
Due after ten years	22,277
Total debt securities	25,433	$ 25,009
AFS, Fair Value
Due in one year or less	479
Due after one year through five years	1,991
Due after five years through ten years	547
Due after ten years	21,269
Total debt securities	24,286	24,547
HTM, Amortized Cost
Due in one year or less	1
Due after one year through five years	3,190
Due after five years through ten years	687
Due after ten years	17,204
Total debt securities	21,082	23,027
HTM, Fair Value
Due in one year or less	1
Due after one year through five years	3,109
Due after five years through ten years	660
Due after ten years	16,493
Total debt securities	$ 20,263	$ 22,837