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Securities - Amortized Cost and Fair Value (Details) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Securities Available-for-Sale |  |  |
Amortized Cost | $ 546,867 | $ 537,645 |
Gross Unrealized Gains | 3,229 | 8,467 |
Gross Unrealized Losses | (8,848) | (4,673) |
Fair Value | 541,248 | 541,439 |
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value | 10,965 |  |
Other disclosures |  |  |
Purchases | 75,044 | 343,470 | $ 210,681
Sales | 94,663 | 232,462 | $ 306,400
Securities pledged to secure deposits and for other purposes | $ 318,400 | 301,000 |
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 |  |
U.S. Treasury |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | $ 4,006 | 4,002 |
Gross Unrealized Losses | (83) | (55) |
Fair Value | 3,923 | 3,947 |
US Treasury and Government |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 11,112 |  |
Gross Unrealized Losses | (161) |  |
Fair Value | 10,951 |  |
U.S. government agencies |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost |  | 13,062 |
Gross Unrealized Gains |  | 8 |
Gross Unrealized Losses |  | (9) |
Fair Value |  | 13,061 |
U.S. government agencies mortgage-backed |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 14,407 | 12,372 |
Gross Unrealized Gains | 45 | 7 |
Gross Unrealized Losses | (377) | (165) |
Fair Value | 14,075 | 12,214 |
States and political subdivisions |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 277,112 | 272,240 |
Gross Unrealized Gains | 1,916 | 7,116 |
Gross Unrealized Losses | (4,961) | (1,264) |
Fair Value | 274,067 | 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 | 66,494 | 66,892 |
Gross Unrealized Gains | 79 | 202 |
Gross Unrealized Losses | (2,144) | (1,155) |
Fair Value | 64,429 | 65,939 |
Asset-backed securities |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 108,574 | 113,983 |
Gross Unrealized Gains | 1,165 | 862 |
Gross Unrealized Losses | (225) | (1,913) |
Fair Value | 109,514 | 112,932 |
FFEL |  |  |
Securities Available-for-Sale |  |  |
Fair Value | $ 93,100 |  |
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 | $ 65,162 | 54,271 |
Gross Unrealized Gains | 24 | 251 |
Gross Unrealized Losses | (897) | (101) |
Fair Value | 64,289 | $ 54,421 |
GCO Education Loan Funding Corp |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 27,739 |  |
Fair Value | $ 27,763 |  |
GCO Education Loan Funding Corp | Stockholders' Equity |  |  |
Other disclosures |  |  |
Credit risk as a percentage of benchmark | 1.00% |  |
Towd Point Mortgage Trust |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | $ 34,308 |  |
Fair Value | 33,318 |  |
Student Marketing Association [Member] |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 25,808 |  |
Fair Value | 26,119 |  |
Stockholders' Equity | Asset-backed securities | Credit Concentration Risk |  |  |
Securities Available-for-Sale |  |  |
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value | 27,739 |  |
Stockholders' Equity | Towd Point Mortgage Trust | Asset-backed securities | Credit Concentration Risk |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 34,308 |  |
Fair Value | 33,318 |  |
Stockholders' Equity | Student Marketing Association [Member] | Asset-backed securities | Credit Concentration Risk |  |  |
Securities Available-for-Sale |  |  |
Amortized Cost | 25,808 |  |
Fair Value | $ 26,119 |  |
Minimum | Stockholders' Equity | Towd Point Mortgage Trust | Asset-backed securities | Credit Concentration Risk |  |  |
Other disclosures |  |  |
Credit risk as a percentage of benchmark | 10.00% |  |
Minimum | Stockholders' Equity | Student Marketing Association [Member] | Asset-backed securities | Credit Concentration Risk |  |  |
Other disclosures |  |  |
Credit risk as a percentage of benchmark | 10.00% |  |

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