Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 230
Amortized Cost of Securities Available for sale, Due after one through five years	19,818
Amortized Cost of Securities Available for sale, Due after five through ten years	17,312
Amortized Cost of Securities Available for sale, Due after ten years	3,301
Amortized Cost	40,661
Fair Value of Securities Available for sale, Due in one year or less	247
Fair Value of Securities Available for sale, Due after one through five years	20,076
Fair Value of Securities Available for sale, Due after five through ten years	17,128
Fair Value of Securities Available for sale, Due after ten years	3,143
Fair Value	40,594	$ 27,556
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	93
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,330
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	2,000
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	8,423	7,595
Fair Value of Held-to-Maturity Securities, Due in one year or less	95
Fair Value of Held-to-Maturity Securities, Due after one through five years	6,452
Fair Value of Held-to-Maturity Securities, Due after five through ten years	2,117
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 8,664	$ 8,023