Investment Securities - Summary (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Accrued interest available-for-sale debt securities	$ 11	$ 5
Debt securities
Cost	2,184
Estimated Fair Value	2,168	2,120
Equity securities
Cost	59	60
Gross Unrealized Gains	17	16
Gross Unrealized Losses	(12)	(10)
Estimated Fair Value	64	66
Total Cost	2,243	2,198
Total Gross Unrealized Gains	17	17
Total Gross Unrealized Losses	(28)	(29)
Total Estimated Fair Value	2,232	2,186
State and municipal obligations
Debt securities
Cost	60	61
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7)	(6)
Estimated Fair Value	53	55
U.S. Government agency obligations
Debt securities
Cost	4	4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	4	4
U.S. Government treasury obligations
Debt securities
Cost	1,219	1,217
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(8)	(12)
Estimated Fair Value	1,211	1,206
Mortgage-backed securities
Debt securities
Cost	12	12
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(1)
Estimated Fair Value	11	11
Foreign government bonds and obligations
Debt securities
Cost	815	770
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	815	770
Other
Debt securities
Cost	74	74
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 74	$ 74