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Securities - Amortized Cost and Fair Value (Details) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 USD ($) item | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) item | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | $ 550,396 | $ 550,396 | $ 550,396 | $ 550,396 | $ 537,645
Gross Unrealized Gains | 2,955 |  | 2,955 |  | 8,467
Gross Unrealized Losses | (9,707) |  | (9,707) |  | (4,673)
Fair Value | 543,644 | 543,644 | 543,644 | 543,644 | 541,439
Other disclosures |  |  |  |  |
Purchases |  |  | 54,550 | 205,755 |
Sales | $ 90,224 | $ 36,468 | $ 92,746 | $ 100,856 |
Number of originators | item | 3 |  | 3 |  |
Securities pledged to secure deposits and for other purposes | $ 311,400 |  | $ 311,400 |  | 301,000
U.S. Treasury |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 4,004 |  | 4,004 |  | 4,002
Gross Unrealized Losses | (128) |  | (128) |  | (55)
Fair Value | 3,876 |  | 3,876 |  | 3,947
U.S. government agencies |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 12,369 |  | 12,369 |  | 13,062
Gross Unrealized Gains |  |  |  |  | 8
Gross Unrealized Losses | (153) |  | (153) |  | (9)
Fair Value | 12,216 |  | 12,216 |  | 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 | 279,007 |  | 279,007 |  | 272,240
Gross Unrealized Gains | 1,640 |  | 1,640 |  | 7,116
Gross Unrealized Losses | (4,535) |  | (4,535) |  | (1,264)
Fair Value | 276,112 |  | 276,112 |  | 278,092
Corporate bonds |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 685 |  | 685 |  | 823
Gross Unrealized Gains | 21 |  | 21 |  | 21
Gross Unrealized Losses | (6) |  | (6) |  | (11)
Fair Value | 700 |  | 700 |  | 833
Collateralized mortgage obligations |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 63,778 |  | 63,778 |  | 66,892
Gross Unrealized Gains | 60 |  | 60 |  | 202
Gross Unrealized Losses | (2,406) |  | (2,406) |  | (1,155)
Fair Value | 61,432 |  | 61,432 |  | 65,939
Asset-backed securities |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 110,053 |  | 110,053 |  | 113,983
Gross Unrealized Gains | 1,011 |  | 1,011 |  | 862
Gross Unrealized Losses | (1,801) |  | (1,801) |  | (1,913)
Fair Value | 109,263 |  | 109,263 |  | 112,932
FFEL |  |  |  |  |
Other disclosures |  |  |  |  |
Asset-backed securities | 92,500 |  | $ 92,500 |  |
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 | 10,900 |  | $ 10,900 |  |
Percentage of total added credit enhancement in the form of overcollaterization and/or subordination of outstanding principal |  |  | 11.44 |  |
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 | 66,489 |  | $ 66,489 |  | 54,271
Gross Unrealized Gains | 223 |  | 223 |  | 251
Gross Unrealized Losses | (74) |  | (74) |  | (101)
Fair Value | 66,638 |  | 66,638 |  | $ 54,421
U.S. government agency mortgage-backed |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 14,011 |  | 14,011 |  |
Gross Unrealized Losses | (604) |  | (604) |  |
Fair Value | 13,407 |  | 13,407 |  |
GCO Education Loan Funding Corp |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 27,685 |  | 27,685 |  |
Fair Value | 26,754 |  | 26,754 |  |
Towd Point Mortgage Trust |  |  |  |  |
Securities Available-for-Sale |  |  |  |  |
Amortized Cost | 28,966 |  | 28,966 |  |
Fair Value | 28,080 |  | $ 28,080 |  |
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 | 25,780 |  | $ 25,780 |  |
Fair Value | $ 26,176 |  | $ 26,176 |  |
Stockholders' Equity | GCO Education Loan Funding Corp |  |  |  |  |
Other disclosures |  |  |  |  |
Credit risk as a percentage of benchmark |  |  | 10.00% |  |

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