Investment Securities - Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Sep. 30, 2023	Dec. 31, 2022
Available-for-Sale [Abstract]
Amortized Cost	$ 22,415
Estimated Fair Value	19,483	$ 20,171
ACL	0
Debt Securities, Held-to-Maturity, Maturity [Abstract]
Amortized Cost	677,578	685,728
Estimated Fair Value	547,290		$ 547,290	$ 568,128
Debt Securities, Available-for-Sale and Held-to-Maturity, after Allowance for Credit Loss [Abstract]
Total Amortized Cost	699,993	708,291
Total Gross Unrealized Gains	36	68
Total Gross Unrealized Losses	(133,256)	(120,060)
Total Estimated Fair Value	566,773	588,299
U.S. government-sponsored mortgage-backed securities
Available-for-Sale [Abstract]
Amortized Cost	22,415	22,563
Gross Unrealized Losses	(2,932)	(2,392)
Estimated Fair Value	19,483	20,171
Debt Securities, Held-to-Maturity, Maturity [Abstract]
Amortized Cost	677,578	685,728
Gross Unrealized Gains	36	68
Gross Unrealized Losses	(130,324)	(117,668)
Estimated Fair Value	$ 547,290	$ 568,128