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Investments - Maturity Table (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 1,986,957 | $ 485,756
Estimated Fair Value | 1,996,634 | $ 487,669
Maturity 2021 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 454,193 |
Estimated Fair Value | 455,185 |
Maturity 2022 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 978,017 |
Estimated Fair Value | 979,802 |
Maturity 2023 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 164,680 |
Estimated Fair Value | 165,299 |
Maturity 2038 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 170 |
Estimated Fair Value | 195 |
Maturity 2039 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,032 |
Estimated Fair Value | 2,249 |
Maturity 2042 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,767 |
Estimated Fair Value | 4,769 |
Maturity 2043 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,745 |
Estimated Fair Value | 9,153 |
Maturity 2044 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 11,586 |
Estimated Fair Value | 12,139 |
Maturity 2045 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 9,739 |
Estimated Fair Value | 10,133 |
Maturity 2046 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,292 |
Estimated Fair Value | 15,748 |
Maturity 2047 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 25,056 |
Estimated Fair Value | 25,574 |
Maturity 2048 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 6,466 |
Estimated Fair Value | 6,743 |
Maturity 2049 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 44,711 |
Estimated Fair Value | 46,681 |
Maturity 2050 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 192,706 |
Estimated Fair Value | 194,056 |
Maturity 2054 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 68,797 |
Estimated Fair Value | $ 68,908 |

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