Securities - Amortized Cost and Fair Value (Details) $ in Thousands	12 Months Ended
Dec. 31, 2021 USD ($) item	Dec. 31, 2020 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 1,677,016	$ 471,961
Gross Unrealized Gains	20,587	26,735
Gross Unrealized Losses	(5,115)	(2,518)
Debt Securities, Available-for-sale, Total	1,692,488	496,178
Accrued interest receivable	4,300	2,700
FHLB and FRB Stock
FHLBC stock	7,100	3,700
FRB stock	6,200	6,200
Securities pledged to secure deposits and for other purposes	501,300	335,800
Other disclosures
Debt Securities, Available-for-sale, Total	$ 1,692,488	496,178
Number of securities issued from originators	0
Number of originators \| item	1
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
US Treasury Securities
Securities Available-for-Sale
Amortized Cost	$ 202,251	4,014
Gross Unrealized Gains	125	103
Gross Unrealized Losses	(37)
Debt Securities, Available-for-sale, Total	202,339	4,117
Other disclosures
Debt Securities, Available-for-sale, Total	202,339	4,117
US Government Corporations and Agencies Securities
Securities Available-for-Sale
Amortized Cost	62,587	6,811
Gross Unrealized Losses	(699)	(154)
Debt Securities, Available-for-sale, Total	61,888	6,657
Other disclosures
Debt Securities, Available-for-sale, Total	61,888	6,657
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	172,016	16,098
Gross Unrealized Gains	856	1,112
Gross Unrealized Losses	(570)	(1)
Debt Securities, Available-for-sale, Total	172,302	17,209
Other disclosures
Debt Securities, Available-for-sale, Total	172,302	17,209
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	240,793	229,352
Gross Unrealized Gains	16,344	21,269
Gross Unrealized Losses	(672)	(1,362)
Debt Securities, Available-for-sale, Total	256,465	249,259
Other disclosures
Debt Securities, Available-for-sale, Total	256,465	249,259
Corporate bonds
Securities Available-for-Sale
Amortized Cost	10,000
Gross Unrealized Losses	(113)
Debt Securities, Available-for-sale, Total	9,887
Other disclosures
Debt Securities, Available-for-sale, Total	9,887
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost	673,238	53,999
Gross Unrealized Gains	2,014	2,866
Gross Unrealized Losses	(2,285)	(280)
Debt Securities, Available-for-sale, Total	672,967	56,585
Other disclosures
Debt Securities, Available-for-sale, Total	672,967	56,585
Asset-backed Securities
Securities Available-for-Sale
Amortized Cost	236,293	130,959
Gross Unrealized Gains	1,245	1,370
Gross Unrealized Losses	(661)	(511)
Debt Securities, Available-for-sale, Total	236,877	131,818
Other disclosures
Debt Securities, Available-for-sale, Total	236,877	131,818
FFEL
Securities Available-for-Sale
Debt Securities, Available-for-sale, Total	194,400
Other disclosures
Debt Securities, Available-for-sale, Total	$ 194,400
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	97
Collateralized loan obligations.
Securities Available-for-Sale
Amortized Cost	$ 79,838	30,728
Gross Unrealized Gains	3	15
Gross Unrealized Losses	(78)	(210)
Debt Securities, Available-for-sale, Total	79,763	30,533
Other disclosures
Debt Securities, Available-for-sale, Total	$ 79,763	$ 30,533
Loan receivables \| GCO Education Loan Funding Corp \| Customer Concentration Risk
Other disclosures
Percentage of concentration risk	1.00%

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Represents the percent of insured loans used as base for loans guaranteed by US Department of Education.

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Reflects percentage of outstanding principal amount of loans guaranteed by US Department of Education.

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Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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