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Investments - Maturity Table (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,116,279 | $ 1,986,957
Estimated Fair Value | 2,115,885 | $ 1,996,634
2021 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 293,759 |
Estimated Fair Value | 294,232 |
2022 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,018,268 |
Estimated Fair Value | 1,019,960 |
2023 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 148,822 |
Estimated Fair Value | 149,166 |
2024 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 100,000 |
Estimated Fair Value | 99,925 |
2038 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 76 |
Estimated Fair Value | 88 |
2039 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,601 |
Estimated Fair Value | 1,773 |
2042 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,201 |
Estimated Fair Value | 4,206 |
2043 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 7,415 |
Estimated Fair Value | 7,711 |
2044 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 10,133 |
Estimated Fair Value | 10,610 |
2045 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,870 |
Estimated Fair Value | 9,179 |
2046 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 14,414 |
Estimated Fair Value | 14,797 |
2047 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 18,646 |
Estimated Fair Value | 18,992 |
2048 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,362 |
Estimated Fair Value | 4,545 |
2049 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 36,368 |
Estimated Fair Value | 37,683 |
2050 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 174,919 |
Estimated Fair Value | 170,740 |
2051 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 75,449 |
Estimated Fair Value | 73,143 |
2053 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 134,414 |
Estimated Fair Value | 134,289 |
2054 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 64,562 |
Estimated Fair Value | $ 64,846 |

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