Securities - Amortized Cost and Fair Value (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Securities Available-for-Sale
Amortized Cost	$ 475,146	$ 546,867
Gross Unrealized Gains	13,285	3,229
Gross Unrealized Losses	(3,783)	(8,848)
Fair Value	484,648	541,248
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value	8,601	10,965
Other disclosures
Purchases	159,544	75,044	$ 343,470
Sales	191,298	94,663	$ 232,462
Securities pledged to secure deposits and for other purposes	$ 320,800	318,400
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
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
US Treasury Securities [Member]
Securities Available-for-Sale
Amortized Cost	$ 4,010	4,006
Gross Unrealized Gains	26
Gross Unrealized Losses		(83)
Fair Value	4,036	3,923
US Treasury and Government
Securities Available-for-Sale
Amortized Cost	8,502
Gross Unrealized Losses	(165)
Fair Value	8,337
US Government Corporations and Agencies Securities [Member]
Securities Available-for-Sale
Amortized Cost		11,112
Gross Unrealized Losses		(161)
Fair Value		10,951
U.S. government agencies mortgage-backed
Securities Available-for-Sale
Amortized Cost	16,164	14,407
Gross Unrealized Gains	443	45
Gross Unrealized Losses	(19)	(377)
Fair Value	16,588	14,075
US States and Political Subdivisions Debt Securities [Member]
Securities Available-for-Sale
Amortized Cost	240,399	277,112
Gross Unrealized Gains	11,207	1,916
Gross Unrealized Losses	(2,431)	(4,961)
Fair Value	249,175	274,067
Collateralized Mortgage Obligations [Member]
Securities Available-for-Sale
Amortized Cost	57,059	66,494
Gross Unrealized Gains	963	79
Gross Unrealized Losses	(38)	(2,144)
Fair Value	57,984	64,429
Asset-backed Securities [Member]
Securities Available-for-Sale
Amortized Cost	82,114	108,574
Gross Unrealized Gains	617	1,165
Gross Unrealized Losses	(887)	(225)
Fair Value	81,844	109,514
FFEL
Securities Available-for-Sale
Fair Value	$ 54,900
Other disclosures
Percentage of outstanding principal amount of loans guaranteed by US Department of Education	97.00
Collateralized loan obligations
Securities Available-for-Sale
Amortized Cost	$ 66,898	65,162
Gross Unrealized Gains	29	24
Gross Unrealized Losses	(243)	(897)
Fair Value	66,684	$ 64,289
GCO Education Loan Funding Corp [Member]
Securities Available-for-Sale
Amortized Cost	27,873
Fair Value	$ 27,470
GCO Education Loan Funding Corp [Member] \| Stockholders' Equity, Total [Member]
Other disclosures
Credit risk as a percentage of benchmark	1.00%
Towd Point Mortgage Trust
Securities Available-for-Sale
Amortized Cost	$ 33,551
Fair Value	$ 34,322

Represents the percent of insured loans used as base for loans guaranteed by US Department of Education.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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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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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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The cash outflow to acquire debt and equity securities not classified as either held-to-maturity securities or trading securities which would be classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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