Investment Securities - Amortized Cost and Estimated Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Due within one year	$ 1
Due from one to five years	1,345
Due from five to ten years	1,486
Due after ten years	12,220
Total debt securities available for sale, net of allowance	15,052	$ 10,402
Mortgage- Related Securities
Debt Securities, Available-for-sale [Line Items]
Due within one year	1
Due from one to five years	122
Due from five to ten years	240
Due after ten years	11,964
Total debt securities available for sale, net of allowance	12,327	8,573
U.S. Government and GSE Obligations
Debt Securities, Available-for-sale [Line Items]
Due within one year	0
Due from one to five years	1,080
Due from five to ten years	1,220
Due after ten years	0
Total debt securities available for sale, net of allowance	2,300
Corporate and Other Bonds
Debt Securities, Available-for-sale [Line Items]
Due within one year	0
Due from one to five years	143
Due from five to ten years	26
Due after ten years	38
Total debt securities available for sale, net of allowance	207
Asset-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Due within one year	0
Due from one to five years	0
Due from five to ten years	0
Due after ten years	218
Total debt securities available for sale, net of allowance	$ 218	$ 236