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Securities - Amortized Cost and Fair Value (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 533,116		$ 545,433
Gross Unrealized Gains	6,824		1,011
Gross Unrealized Losses	(6,456)		(14,606)
Fair Value	533,484		531,838
Other disclosures
Purchases	246,971	$ 153,252
Sales	$ 152,476	$ 271,374
Number of originators | item	3
U.S. Treasury
Securities Available-for-Sale
Amortized Cost	$ 4,001
Gross Unrealized Losses	(11)
Fair Value	3,990
U.S. government agencies
Securities Available-for-Sale
Amortized Cost	13,475		42,511
Gross Unrealized Gains	15
Gross Unrealized Losses	(39)		(977)
Fair Value	13,451		41,534
States and political subdivisions
Securities Available-for-Sale
Amortized Cost	224,648		68,718
Gross Unrealized Gains	5,173		258
Gross Unrealized Losses	(789)		(273)
Fair Value	229,032		68,703
Corporate bonds
Securities Available-for-Sale
Amortized Cost	10,823		10,957
Gross Unrealized Gains	20		9
Gross Unrealized Losses	(266)		(336)
Fair Value	10,577		10,630
Collateralized mortgage obligations
Securities Available-for-Sale
Amortized Cost	81,693		174,352
Gross Unrealized Gains	228		374
Gross Unrealized Losses	(1,535)		(3,799)
Fair Value	80,386		170,927
Asset-backed securities
Securities Available-for-Sale
Amortized Cost	134,542		146,391
Gross Unrealized Gains	865		341
Gross Unrealized Losses	(3,648)		(8,325)
Fair Value	131,759		138,407
FFEL
Other disclosures
Asset-backed securities	$ 110,600
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	97.00
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal	10.42
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	$ 52,803		102,504
Gross Unrealized Gains	505		29
Gross Unrealized Losses	(49)		(896)
Fair Value	53,259		$ 101,637
U.S. government agency mortgage-backed
Securities Available-for-Sale
Amortized Cost	11,131
Gross Unrealized Gains	18
Gross Unrealized Losses	(119)
Fair Value	11,030
GCO Education Loan Funding Corp
Securities Available-for-Sale
Amortized Cost	27,555
Fair Value	$ 26,505
GCO Education Loan Funding Corp | Stockholders' Equity
Other disclosures
Credit risk as a percentage of benchmark	10.00%
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	$ 29,544
Fair Value	$ 29,445
Towd Point Mortgage Trust | Stockholders' Equity
Other disclosures
Credit risk as a percentage of benchmark	10.00%
Student Loan Marketing Assocation
Securities Available-for-Sale
Amortized Cost	$ 25,654
Fair Value	$ 25,803

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