Securities - Amortized Cost and Fair Value (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 471,961	$ 475,146
Gross Unrealized Gains	26,735	13,285
Gross Unrealized Losses	(2,518)	(3,783)
Fair Value	496,178	484,648
Accrued interest receivable	2,700	3,200
Other disclosures
Purchases	65,229	159,544	$ 75,044
Sales	18,006	191,298	94,663
Securities pledged to secure deposits and for other purposes	335,800	$ 320,800	$ 320,800
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 [Member]
Securities Available-for-Sale
Amortized Cost	4,014	$ 4,010
Gross Unrealized Gains	103	26
Fair Value	4,117	4,036
US Treasury and Government
Securities Available-for-Sale
Amortized Cost	6,811
Gross Unrealized Losses	(154)
Fair Value	6,657
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost		8,502
Gross Unrealized Losses		(165)
Fair Value		8,337
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	16,098	16,164
Gross Unrealized Gains	1,112	443
Gross Unrealized Losses	(1)	(19)
Fair Value	17,209	16,588
US States and Political Subdivisions Debt Securities [Member]
Securities Available-for-Sale
Amortized Cost	229,352	240,399
Gross Unrealized Gains	21,269	11,207
Gross Unrealized Losses	(1,362)	(2,431)
Fair Value	249,259	249,175
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost	53,999	57,059
Gross Unrealized Gains	2,866	963
Gross Unrealized Losses	(280)	(38)
Fair Value	56,585	57,984
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost	130,959	82,114
Gross Unrealized Gains	1,370	617
Gross Unrealized Losses	(511)	(887)
Fair Value	131,818	81,844
FFEL
Securities Available-for-Sale
Fair Value	105,000	$ 105,000
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		97
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost	30,728	$ 66,898
Gross Unrealized Gains	15	29
Gross Unrealized Losses	(210)	(243)
Fair Value	30,533	$ 66,684
GCO Education Loan Funding Corp [Member] \| Stockholders' Equity, Total [Member]
Other disclosures
Credit risk as a percentage of benchmark		1.00%
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	33,198
Fair Value	$ 35,479

Represents the percent of insured loans used as base for 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 investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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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The cash outflow to acquire debt and equity securities not classified as either held-to-maturity securities or trading securities which would be classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity.

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