Securities - Securities by Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost of Securities Available for sale, Due in one year or less	$ 271
Amortized Cost of Securities Available for sale, Due after one through five years	15,750
Amortized Cost of Securities Available for sale, Due after five through ten years	5,071
Amortized Cost of Securities Available for sale, Due after ten years	10
Amortized Cost	21,102	$ 19,917
Fair Value of Securities Available for sale, Due in one year or less	276
Fair Value of Securities Available for sale, Due after one through five years	15,510
Fair Value of Securities Available for sale, Due after five through ten years	5,057
Fair Value of Securities Available for sale, Due after ten years	11
Total Fair Value of Securities Available For Sale	20,854	19,428
Amortized Cost of Held-to-Maturity Securities, Due in one year or less	52
Amortized Cost of Held-to-Maturity Securities, Due after one through five years	6,791
Amortized Cost of Held-to-Maturity Securities, Due after five through ten years	4,391
Amortized Cost of Held-to-Maturity Securities, Due after ten years	0
Amortized Cost	11,234	11,519
Fair Value of Held-to-Maturity Securities, Due in one year or less	52
Fair Value of Held-to-Maturity Securities, Due after one through five years	6,647
Fair Value of Held-to-Maturity Securities, Due after five through ten years	4,298
Fair Value of Held-to-Maturity Securities, Due after ten years	0
Total Fair Value of Held-to-Maturity Securities	$ 10,997	$ 11,122