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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Securities Available-for-Sale
Amortized Cost		$ 446,544		$ 475,146
Gross Unrealized Gains		14,716		13,285
Gross Unrealized Losses		(11,566)		(3,783)
Fair Value	$ 449,694	449,694	$ 449,694	484,648
Accrued interest receivable		3,000		3,200
Other disclosures
Purchases	46,176	6,100
Sales	$ 81,524	$ 18,126	$ 81,524
Number of originators | item		2
Securities pledged to secure deposits and for other purposes		$ 320,300		320,800
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost		4,011		4,010
Gross Unrealized Gains		141		26
Fair Value		4,152		4,036
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost		7,870		8,502
Gross Unrealized Losses		(147)		(165)
Fair Value		7,723		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		244,429		240,399
Gross Unrealized Gains		12,353		11,207
Gross Unrealized Losses		(1,687)		(2,431)
Fair Value		255,095		249,175
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost		56,104		57,059
Gross Unrealized Gains		709		963
Gross Unrealized Losses		(3,410)		(38)
Fair Value		53,403		57,984
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost		82,005		82,114
Gross Unrealized Gains		210		617
Gross Unrealized Losses		(4,488)		(887)
Fair Value		77,727		81,844
FFEL
Other disclosures
Asset-backed securities		$ 55,800
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		97.00
Total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal		$ 4,800
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal		8.52
FFEL | Reimbursement requests greater than 5 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		85.00
Percent of insured loan		5.00
FFEL | Reimbursement requests greater than 9 %
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education		75.00
Percent of insured loan		9.00
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost		$ 36,104		66,898
Gross Unrealized Gains		69		29
Gross Unrealized Losses		(1,834)		(243)
Fair Value		34,339		$ 66,684
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Securities Available-for-Sale
Amortized Cost		16,021
Gross Unrealized Gains		1,234
Fair Value		17,255
GCO Education Loan Funding Corp [Member]
Securities Available-for-Sale
Amortized Cost		27,911
Fair Value		26,501
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost		33,360
Fair Value		$ 31,996
Towd Point Mortgage Trust | Stockholders' Equity, Total [Member]
Other disclosures
Credit risk as a percentage of benchmark		10.00%
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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