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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Securities Available-for-Sale
Amortized Cost		$ 433,267		$ 475,146
Gross Unrealized Gains		19,945		13,285
Gross Unrealized Losses		(5,776)		(3,783)
Fair Value		447,436		484,648
Accrued interest receivable		2,900		3,200
Other disclosures
Purchases		12,800	$ 82,930
Sales	$ 39,072	$ 18,006	$ 120,596
Number of originators | item		1
Securities pledged to secure deposits and for other purposes		$ 329,500		320,800
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost		4,012		4,010
Gross Unrealized Gains		135		26
Fair Value		4,147		4,036
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost		7,450		8,502
Gross Unrealized Losses		(174)		(165)
Fair Value		7,276		8,337
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost				16,164
Gross Unrealized Gains				443
Gross Unrealized Losses				(19)
Fair Value				16,588
US States and Political Subdivisions Debt Securities [Member]
Securities Available-for-Sale
Amortized Cost		235,111		240,399
Gross Unrealized Gains		16,388		11,207
Gross Unrealized Losses		(1,135)		(2,431)
Fair Value		250,364		249,175
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost		55,391		57,059
Gross Unrealized Gains		1,672		963
Gross Unrealized Losses		(950)		(38)
Fair Value		56,113		57,984
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost		82,004		82,114
Gross Unrealized Gains		430		617
Gross Unrealized Losses		(2,408)		(887)
Fair Value		80,026		81,844
FFEL
Other disclosures
Asset-backed securities		$ 55,900
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		97
Total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal		$ 4,900
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal		8.71
FFEL | 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
FFEL | 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
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost		$ 33,794		66,898
Gross Unrealized Gains		46		29
Gross Unrealized Losses		(1,109)		(243)
Fair Value		32,731		$ 66,684
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Securities Available-for-Sale
Amortized Cost		15,505
Gross Unrealized Gains		1,274
Fair Value		16,779
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost		33,298
Fair Value		$ 34,394
Stockholders' Equity, Total [Member] | GCO Education Loan Funding Corp [Member]
Other disclosures
Credit risk as a percentage of benchmark		10.00%

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