Investment Securities - Gross Unrealized Losses (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Estimated Fair Value
Less than 12 months	$ 0	$ 0
12 months or more	1,134	1,154
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	(16)	(19)
State and municipal obligations
Estimated Fair Value
Less than 12 months	0	0
12 months or more	30	33
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	(7)	(6)
U.S. Government treasury obligations
Estimated Fair Value
Less than 12 months	0	0
12 months or more	1,095	1,114
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	(8)	(12)
Mortgage-backed securities
Estimated Fair Value
Less than 12 months	0	0
12 months or more	9	7
Gross Unrealized Losses
Less than 12 months	0	0
12 months or more	$ (1)	$ (1)