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Investments - Schedule of Maturity (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 185,749 | $ 247,607
Available-for-sale investments | 178,145 | $ 244,088
2038 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 269 |
Available-for-sale investments | 284 |
2039 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,099 |
Available-for-sale investments | 3,203 |
2042 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 10,099 |
Available-for-sale investments | 9,381 |
2043 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,371 |
Available-for-sale investments | 14,823 |
2044 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 24,183 |
Available-for-sale investments | 23,464 |
2045 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 28,117 |
Available-for-sale investments | 26,883 |
2046 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 41,807 |
Available-for-sale investments | 39,789 |
2047 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 59,890 |
Available-for-sale investments | 57,416 |
2048 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,914 |
Available-for-sale investments | $ 2,902 |

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