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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 551,903	$ 551,903	$ 551,903	$ 551,903	$ 537,645
Gross Unrealized Gains	1,669		1,669		8,467
Gross Unrealized Losses	(11,234)		(11,234)		(4,673)
Fair Value	542,338	542,338	542,338	542,338	541,439
Other disclosures
Purchases			71,488	246,971
Sales	$ 1,917	$ 51,620	$ 94,663	$ 152,476
Number of originators | item	3		3
Securities pledged to secure deposits and for other purposes	$ 311,200		$ 311,200		301,000
U.S. Treasury
Securities Available-for-Sale
Amortized Cost	4,005		4,005		4,002
Gross Unrealized Losses	(151)		(151)		(55)
Fair Value	3,854		3,854		3,947
U.S. government agencies
Securities Available-for-Sale
Amortized Cost	11,881		11,881		13,062
Gross Unrealized Gains					8
Gross Unrealized Losses	(178)		(178)		(9)
Fair Value	11,703		11,703		13,061
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost					12,372
Gross Unrealized Gains					7
Gross Unrealized Losses					(165)
Fair Value					12,214
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	278,604		278,604		272,240
Gross Unrealized Gains	398		398		7,116
Gross Unrealized Losses	(6,738)		(6,738)		(1,264)
Fair Value	272,264		272,264		278,092
Corporate bonds
Securities Available-for-Sale
Amortized Cost					823
Gross Unrealized Gains					21
Gross Unrealized Losses					(11)
Fair Value					833
Collateralized mortgage obligations
Securities Available-for-Sale
Amortized Cost	67,762		67,762		66,892
Gross Unrealized Gains	118		118		202
Gross Unrealized Losses	(2,920)		(2,920)		(1,155)
Fair Value	64,960		64,960		65,939
Asset-backed securities
Securities Available-for-Sale
Amortized Cost	108,622		108,622		113,983
Gross Unrealized Gains	991		991		862
Gross Unrealized Losses	(440)		(440)		(1,913)
Fair Value	109,173		109,173		112,932
FFEL
Other disclosures
Asset-backed securities	92,600		$ 92,600
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		$ 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	65,555		$ 65,555		54,271
Gross Unrealized Gains	162		162		251
Gross Unrealized Losses	(99)		(99)		(101)
Fair Value	65,618		65,618		$ 54,421
U.S. government agency mortgage-backed
Securities Available-for-Sale
Amortized Cost	15,474		15,474
Gross Unrealized Losses	(708)		(708)
Fair Value	14,766		14,766
GCO Education Loan Funding Corp
Securities Available-for-Sale
Amortized Cost	27,711		27,711
Fair Value	27,788		27,788
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	34,390		34,390
Fair Value	33,181		$ 33,181
Towd Point Mortgage Trust | Stockholders' Equity
Other disclosures
Credit risk as a percentage of benchmark			10.00%
Student Marketing Association [Member]
Securities Available-for-Sale
Amortized Cost	25,798		$ 25,798
Fair Value	$ 26,134		$ 26,134
Stockholders' Equity | GCO Education Loan Funding Corp
Other disclosures
Credit risk as a percentage of benchmark			10.00%

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