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Investments - Maturity Table (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 247,607 | $ 211,406
Mortgage-backed securities | 244,088 | $ 208,603
Maturity 2038 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 272 |
Mortgage-backed securities | 293 |
Maturity 2039 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 3,406 |
Mortgage-backed securities | 3,606 |
Maturity 2042 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,993 |
Mortgage-backed securities | 10,487 |
Maturity 2043 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 37,438 |
Mortgage-backed securities | 37,534 |
Maturity 2044 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 28,301 |
Mortgage-backed securities | 28,206 |
Maturity 2045 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 41,082 |
Mortgage-backed securities | 40,484 |
Maturity 2046 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 48,785 |
Mortgage-backed securities | 47,771 |
Maturity 2047 |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 77,330 |
Mortgage-backed securities | $ 75,707 |

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