Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 116
Amortized Cost of Securities Available for sale, Due after one through five years	12,298
Amortized Cost of Securities Available for sale, Due after five through ten years	7,492
Amortized Cost of Securities Available for sale, Due after ten years	11
Amortized Cost	19,917	$ 18,546
Fair Value of Securities Available for sale, Due in one year or less	116
Fair Value of Securities Available for sale, Due after one through five years	11,949
Fair Value of Securities Available for sale, Due after five through ten years	7,352
Fair Value of Securities Available for sale, Due after ten years	11
Total Fair Value of Securities Available-for-sale	19,428	18,139
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	36
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,402
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	5,081
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	11,519	11,830
Fair Value of Held-to-Maturity Securities, Due in one year or less	36
Fair Value of Held-to-Maturity Securities, Due after one through five years	6,210
Fair Value of Held-to-Maturity Securities, Due after five through ten years	4,876
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 11,122	$ 11,565