Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 5,125
Amortized Cost of Securities Available for sale, Due after one through five years	14,883
Amortized Cost of Securities Available for sale, Due after five through ten years	5,608
Amortized Cost of Securities Available for sale, Due after ten years	438
Amortized Cost	26,054	$ 21,692
Fair Value of Securities Available for sale, Due in one year or less	5,134
Fair Value of Securities Available for sale, Due after one through five years	15,439
Fair Value of Securities Available for sale, Due after five through ten years	5,887
Fair Value of Securities Available for sale, Due after ten years	435
Total Fair Value of Securities Available For Sale	26,895	21,843
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	102
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	5,649
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,620
Amortized Cost of Held-to-Maturity Securities, Due after ten years	13
Amortized Cost	8,384	10,067
Fair Value of Held-to-Maturity Securities, Due in one year or less	104
Fair Value of Held-to-Maturity Securities, Due after one through five years	5,880
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,845
Fair Value of Held-to-Maturity Securities, Due after ten years	13
Total Fair Value of Held-to-Maturity Securities	$ 8,842	$ 10,116