Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 190
Amortized Cost of Securities Available for sale, Due after one through five years	14,990
Amortized Cost of Securities Available for sale, Due after five through ten years	3,358
Amortized Cost of Securities Available for sale, Due after ten years	8
Total Amortized Cost of Securities Available for sale	18,546
Fair Value of Securities Available for sale, Due in one year or less	190
Fair Value of Securities Available for sale, Due after one through five years	14,635
Fair Value of Securities Available for sale, Due after five through ten years	3,305
Fair Value of Securities Available for sale, Due after ten years	9
Total Fair Value of Securities Available-for-sale	18,139	$ 20,212
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	56
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	7,977
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	3,190
Amortized Cost of Held-to-Maturity Securities, Due after ten years	607
Held to maturities, Amortized Cost	11,830	10,232
Fair Value of Held-to-Maturity Securities, Due in one year or less	56
Fair Value of Held-to-Maturity Securities, Due after one through five years	7,776
Fair Value of Held-to-Maturity Securities, Due after five through ten years	3,142
Fair Value of Held-to-Maturity Securities, Due after ten years	591
Total Fair Value of Held-to-Maturity Securities	$ 11,565	$ 10,007