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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 474,856	$ 474,856	$ 474,856	$ 474,856		$ 546,867
Gross Unrealized Gains	15,115		15,115			3,229
Gross Unrealized Losses	(1,549)		(1,549)			(8,848)
Fair Value	488,422	488,422	488,422	488,422		541,248
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value	9,135	10,887	9,135	10,887
Other disclosures
Purchases			136,096	71,488
Sales	$ 57,228	$ 1,917	$ 177,824	$ 94,663
Number of originators | item	2		2
Securities pledged to secure deposits and for other purposes	$ 328,800		$ 328,800			311,200
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost					$ 4,009	4,006
Gross Unrealized Gains					29
Gross Unrealized Losses						(83)
Fair Value					4,038	3,923
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost					9,304	11,112
Gross Unrealized Losses					(161)	(161)
Fair Value					9,143	10,951
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost						14,407
Gross Unrealized Gains						45
Gross Unrealized Losses						(377)
Fair Value						14,075
US States and Political Subdivisions Debt Securities [Member]
Securities Available-for-Sale
Amortized Cost	227,680		227,680			277,112
Gross Unrealized Gains	11,962		11,962			1,916
Gross Unrealized Losses	(915)		(915)			(4,961)
Fair Value	238,727		238,727			274,067
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost	62,965		62,965			66,494
Gross Unrealized Gains	1,216		1,216			79
Gross Unrealized Losses	(60)		(60)			(2,144)
Fair Value	64,121		64,121			64,429
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost	82,106		82,106			108,574
Gross Unrealized Gains	1,243		1,243			1,165
Gross Unrealized Losses	(167)		(167)			(225)
Fair Value	83,182		83,182			109,514
FFEL
Other disclosures
Asset-backed securities	55,700		$ 55,700
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	4,700		$ 4,700
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal			8.30
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	72,454		$ 72,454			65,162
Gross Unrealized Gains	46		46			24
Gross Unrealized Losses	(229)		(229)			(897)
Fair Value	72,271		72,271			$ 64,289
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Securities Available-for-Sale
Amortized Cost					16,338
Gross Unrealized Gains					619
Gross Unrealized Losses					(17)
Fair Value					$ 16,940
GCO Education Loan Funding Corp [Member]
Securities Available-for-Sale
Amortized Cost	27,840		27,840
Fair Value	27,744		27,744
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	33,744		33,744
Fair Value	$ 34,378		$ 34,378
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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Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.

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