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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2024 USD ($) item	Dec. 31, 2023 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 1,247,087	$ 1,276,981
Gross Unrealized Gains	4,146	1,818
Gross Unrealized Losses	(60,379)	(85,970)
Securities available-for-sale	1,190,854	1,192,829
Accrued interest receivable	7,300	6,600
FHLB and FRB Stock
FHLBC stock	15,300	18,500
FRBC stock	14,900	14,900
Securities pledged to secure deposits and for other purposes	$ 755,200	810,200
Other disclosures
Number of securities issued from originators | item	0
Number of originators | item	1
Minimum percentage of securities investment	10.00%
U.S. Treasury
Securities Available-for-Sale
Amortized Cost	$ 193,825	174,602
Gross Unrealized Gains	1,559
Gross Unrealized Losses	(1,196)	(5,028)
Securities available-for-sale	194,188	169,574
U.S. government agencies
Securities Available-for-Sale
Amortized Cost	39,401	60,011
Gross Unrealized Losses	(1,425)	(3,052)
Securities available-for-sale	37,976	56,959
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	105,262	118,492
Gross Unrealized Losses	(8,849)	(12,122)
Securities available-for-sale	96,413	106,370
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	231,506	236,072
Gross Unrealized Gains	1,237	1,325
Gross Unrealized Losses	(7,948)	(10,332)
Securities available-for-sale	224,795	227,065
Collateralized mortgage obligations
Securities Available-for-Sale
Amortized Cost	422,234	442,987
Gross Unrealized Gains	1,027	421
Gross Unrealized Losses	(38,990)	(50,864)
Securities available-for-sale	384,271	392,544
Asset-backed Securities
Securities Available-for-Sale
Amortized Cost	65,716	71,616
Gross Unrealized Gains	142	42
Gross Unrealized Losses	(1,911)	(3,222)
Securities available-for-sale	63,947	68,436
Collateralized loan obligations.
Securities Available-for-Sale
Amortized Cost	189,143	173,201
Gross Unrealized Gains	181	30
Gross Unrealized Losses	(60)	(1,350)
Securities available-for-sale	$ 189,264	$ 171,881

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