Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 268
Amortized Cost of Securities Available for sale, Due after one through five years	15,505
Amortized Cost of Securities Available for sale, Due after five through ten years	14,962
Amortized Cost of Securities Available for sale, Due after ten years	3,740
Amortized Cost	34,475
Fair Value of Securities Available for sale, Due in one year or less	284
Fair Value of Securities Available for sale, Due after one through five years	15,821
Fair Value of Securities Available for sale, Due after five through ten years	14,923
Fair Value of Securities Available for sale, Due after ten years	3,610
Total Fair Value of Securities Available For Sale	34,638	$ 27,556
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	121
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	3,934
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,120
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	6,175	7,595
Fair Value of Held-to-Maturity Securities, Due in one year or less	122
Fair Value of Held-to-Maturity Securities, Due after one through five years	4,085
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,267
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 6,474	$ 8,023