Securities - Schedule of Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
AFS, Amortized Cost
Due in one year or less	$ 336
Due after one year through five years	498
Due after five years through ten years	2,419
Due after ten years	21,756
Total debt securities	25,009
AFS, Fair Value
Due in one year or less	335
Due after one year through five years	496
Due after five years through ten years	2,341
Due after ten years	21,375
Total debt securities	24,547
HTM, Amortized Cost
Due in one year or less	0
Due after one year through five years	2,237
Due after five years through ten years	1,111
Due after ten years	19,679
Amortized Cost	23,027	$ 16,680
HTM, Fair Value
Due in one year or less	0
Due after one year through five years	2,242
Due after five years through ten years	1,103
Due after ten years	19,492
Total debt securities	$ 22,837	$ 16,546