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Investments - Maturity Table (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 182,325 | $ 247,607
Estimated Fair Value | 176,245 | $ 244,088
Maturity 2038 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 265 |
Estimated Fair Value | 279 |
Maturity 2039 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,963 |
Estimated Fair Value | 3,060 |
Maturity 2042 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,689 |
Estimated Fair Value | 8,156 |
Maturity 2043 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 14,192 |
Estimated Fair Value | 13,779 |
Maturity 2044 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 22,132 |
Estimated Fair Value | 21,556 |
Maturity 2045 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 23,918 |
Estimated Fair Value | 23,005 |
Maturity 2046 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 37,939 |
Estimated Fair Value | 36,290 |
Maturity 2047 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 56,376 |
Estimated Fair Value | 54,216 |
Maturity 2048 [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,851 |
Estimated Fair Value | $ 15,904 |

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