Investment Securities - Amortized Cost and Estimated Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investment Securities Available-for-Sale, Amortized Cost
Due in one year or less	$ 6
Due after one year through five years	9
Due after five years through 10 years	119
Due after 10 years	1,635
Amortized Cost	1,769	$ 1,870
Investment Securities Available-for-Sale, Fair Value
Due in one year or less	6
Due after one year through five years	9
Due after five years through 10 years	125
Due after 10 years	1,662
Fair Value	$ 1,802	1,944
Investment Securities Available-for-Sale, Weighted Average Yield
Due in one year or less (as a percent)	2.24%
Due after one year through five years (as a percent)	3.91%
Due after five years through 10 years (as a percent)	3.65%
Due after 10 years (as a percent)	2.23%
Investment Securities Held-to-maturity, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	10
Due after five years through 10 years	3
Due after 10 years	223
Amortized Cost	236	377
Investment Securities Held-to-maturity, Fair Value
Due in one year or less	0
Due after one year through five years	10
Due after five years through 10 years	3
Due after 10 years	231
Fair Value	$ 244	$ 393
Investment Securities Held-to-maturity, Weighted Average Yield
Due in one year or less (as a percent)	0.00%
Due after one year through five years (as a percent)	2.51%
Due after five years through 10 years (as a percent)	1.99%
Due after 10 years (as a percent)	2.52%