Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
AFS, Amortized Cost
Due in one year or less	$ 255
Due after one year through five years	1,178
Due after five years through ten years	258
Due after ten years	33,951
Total debt securities	35,642	$ 25,689
AFS, Fair Value
Due in one year or less	256
Due after one year through five years	1,174
Due after five years through ten years	272
Due after ten years	34,295
Total debt securities	35,997	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	529
Due after ten years	14,939
Total debt securities	18,768	20,552
HTM, Fair Value
Due in one year or less	0
Due after one year through five years	3,342
Due after five years through ten years	530
Due after ten years	15,098
Total debt securities	$ 18,970	$ 20,047