Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
AFS, Amortized Cost
Due in one year or less	$ 346
Due after one year through five years	1,011
Due after five years through ten years	278
Due after ten years	23,967
Total debt securities	25,602	$ 25,689
AFS, Fair Value
Due in one year or less	347
Due after one year through five years	1,002
Due after five years through ten years	280
Due after ten years	24,173
Total debt securities	25,802	25,038
HTM, Amortized Cost
Due in one year or less	0
Due after one year through five years	3,300
Due after five years through ten years	563
Due after ten years	15,624
Total debt securities	19,487	20,552
HTM, Fair Value
Due in one year or less	0
Due after one year through five years	3,332
Due after five years through ten years	564
Due after ten years	15,669
Total debt securities	$ 19,565	$ 20,047