Investment Securities - Amortized Cost and Estimated Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investment Securities Available-for-Sale, Amortized Cost
Due in one year or less	$ 3
Due after one year through five years	11
Due after five years through 10 years	102
Due after 10 years	2,280
Amortized Cost	2,396	$ 2,115
Investment Securities Available-for-Sale, Estimated Fair Value
Due in one year or less	3
Due after one year through five years	11
Due after five years through 10 years	104
Due after 10 years	2,328
Fair Value	$ 2,446	2,116
Investment Securities Available-for-Sale, Weighted-Average Yield
Due in one year or less	2.04%
Due after one year through five years	2.68%
Due after five years through 10 years	4.03%
Due after 10 years	2.42%
Investment Securities Held-to-maturity, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	9
Due after five years through 10 years	8
Due after 10 years	537
Amortized Cost	554	598
Investment Securities Held-to-maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	9
Due after five years through 10 years	9
Due after 10 years	551
Fair Value	$ 569	$ 599
Investment Securities Held-to-maturity, Weighted-Average Yield
Due in one year or less	0.00%
Due after one year through five years	2.49%
Due after five years through 10 years	2.28%
Due after 10 years	2.44%