Investment Securities - Amortized Cost and Estimated Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Investment Securities Available-for-Sale, Amortized Cost
Due in one year or less	$ 4
Due after one year through five years	10
Due after five years through 10 years	110
Due after 10 years	2,144
Amortized Cost	2,268	$ 2,115
Investment Securities Available-for-Sale, Estimated Fair Value
Due in one year or less	4
Due after one year through five years	10
Due after five years through 10 years	112
Due after 10 years	2,222
Fair Value	$ 2,348	2,116
Investment Securities Available-for-Sale, Weighted Average Yield
Due in one year or less (as a percent)	2.07%
Due after one year through five years (as a percent)	2.73%
Due after five years through 10 years (as a percent)	4.19%
Due after 10 years (as a percent)	2.38%
Investment Securities Held-to-maturity, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	8
Due after five years through 10 years	8
Due after 10 years	480
Amortized Cost	496	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	8
Due after 10 years	499
Fair Value	$ 516	$ 599
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)	2.30%
Due after 10 years (as a percent)	2.38%