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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($) item	Dec. 31, 2021 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 1,865,807	$ 1,677,016
Gross Unrealized Gains	5,029	20,587
Gross Unrealized Losses	(54,386)	(5,115)
Debt Securities, Available-for-sale, Total	1,816,450	1,692,488
Accrued interest receivable	5,200	4,300
FHLB and FRB Stock
FHLBC stock	7,100	7,100
FRBC stock	14,900	6,200
Securities pledged to secure deposits and for other purposes	478,200	501,300
Other disclosures
Debt Securities, Available-for-sale, Total	1,816,450	1,692,488
Total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal	$ 20,300
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal	9.29
Number of securities issued from originators | item	0
Minimum percentage of securities investment	10.00%
Reimbursement requests greater than 5 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	85
Percent of insured loan	5
Reimbursement requests greater than 9 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	75
Percent of insured loan	9
U.S. Treasury
Securities Available-for-Sale
Amortized Cost	$ 227,646	202,251
Gross Unrealized Gains	13	125
Gross Unrealized Losses	(7,096)	(37)
Debt Securities, Available-for-sale, Total	220,563	202,339
Other disclosures
Debt Securities, Available-for-sale, Total	220,563	202,339
U.S. government agencies
Securities Available-for-Sale
Amortized Cost	62,029	62,587
Gross Unrealized Losses	(2,993)	(699)
Debt Securities, Available-for-sale, Total	59,036	61,888
Other disclosures
Debt Securities, Available-for-sale, Total	59,036	61,888
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	161,119	172,016
Gross Unrealized Gains	117	856
Gross Unrealized Losses	(8,088)	(570)
Debt Securities, Available-for-sale, Total	153,148	172,302
Other disclosures
Debt Securities, Available-for-sale, Total	153,148	172,302
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	237,988	240,793
Gross Unrealized Gains	4,358	16,344
Gross Unrealized Losses	(5,938)	(672)
Debt Securities, Available-for-sale, Total	236,408	256,465
Other disclosures
Debt Securities, Available-for-sale, Total	236,408	256,465
Corporate bonds
Securities Available-for-Sale
Amortized Cost	10,000	10,000
Gross Unrealized Losses	(317)	(113)
Debt Securities, Available-for-sale, Total	9,683	9,887
Other disclosures
Debt Securities, Available-for-sale, Total	9,683	9,887
Collateralized mortgage obligations
Securities Available-for-Sale
Amortized Cost	720,960	673,238
Gross Unrealized Gains	298	2,014
Gross Unrealized Losses	(24,745)	(2,285)
Debt Securities, Available-for-sale, Total	696,513	672,967
Other disclosures
Debt Securities, Available-for-sale, Total	696,513	672,967
Asset-backed Securities
Securities Available-for-Sale
Amortized Cost	278,941	236,293
Gross Unrealized Gains	240	1,245
Gross Unrealized Losses	(4,240)	(661)
Debt Securities, Available-for-sale, Total	274,941	236,877
Other disclosures
Debt Securities, Available-for-sale, Total	274,941	236,877
FFEL
Securities Available-for-Sale
Debt Securities, Available-for-sale, Total	218,100
Other disclosures
Debt Securities, Available-for-sale, Total	$ 218,100
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	97
Collateralized loan obligations.
Securities Available-for-Sale
Amortized Cost	$ 167,124	79,838
Gross Unrealized Gains	3	3
Gross Unrealized Losses	(969)	(78)
Debt Securities, Available-for-sale, Total	166,158	79,763
Other disclosures
Debt Securities, Available-for-sale, Total	$ 166,158	$ 79,763

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