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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 505,544	$ 505,544	$ 546,867
Gross Unrealized Gains	8,077		3,229
Gross Unrealized Losses	(4,531)		(8,848)
Fair Value	509,090	509,090	541,248
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value	10,851
Other disclosures
Purchases	46,176	23,930
Sales	$ 81,524	$ 2,522
Number of originators | item	3
Securities pledged to secure deposits and for other purposes	$ 321,800		318,400
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost	4,007		4,006
Gross Unrealized Losses	(47)		(83)
Fair Value	3,960		3,923
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost	10,511		11,112
Gross Unrealized Losses	(151)		(161)
Fair Value	10,360		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	276,463		277,112
Gross Unrealized Gains	6,796		1,916
Gross Unrealized Losses	(2,087)		(4,961)
Fair Value	281,172		274,067
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost	65,354		66,494
Gross Unrealized Gains	182		79
Gross Unrealized Losses	(1,206)		(2,144)
Fair Value	64,330		64,429
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost	70,123		108,574
Gross Unrealized Gains	838		1,165
Gross Unrealized Losses	(150)		(225)
Fair Value	70,811		109,514
FFEL
Other disclosures
Asset-backed securities	$ 55,600
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	$ 11,700
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal	12.44
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	$ 63,808		65,162
Gross Unrealized Gains	79		24
Gross Unrealized Losses	(736)		(897)
Fair Value	63,151		$ 64,289
Mortgage Backed Securities, Issued by US Government Agencies [Member]
Securities Available-for-Sale
Amortized Cost	15,278
Gross Unrealized Gains	182
Gross Unrealized Losses	(154)
Fair Value	15,306
GCO Education Loan Funding Corp [Member]
Securities Available-for-Sale
Amortized Cost	27,777
Fair Value	27,731
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	34,118
Fair Value	$ 33,679
Towd Point Mortgage Trust | Stockholders' Equity, Total [Member]
Other disclosures
Credit risk as a percentage of benchmark	10.00%
Student Marketing Association [Member]
Securities Available-for-Sale
Amortized Cost	$ 25,824
Fair Value	$ 26,153
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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