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Investments - Maturity Table (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 2,506,087 | $ 1,986,957
Estimated Fair Value | 2,505,372 | $ 1,996,634
2021 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 16,029 |
Estimated Fair Value | 16,045 |
2022 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 850,772 |
Estimated Fair Value | 851,858 |
2023 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 162,641 |
Estimated Fair Value | 162,906 |
2024 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 448,293 |
Estimated Fair Value | 447,649 |
2026 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 447,886 |
Estimated Fair Value | 447,991 |
2038 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 74 |
Estimated Fair Value | 80 |
2039 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 1,079 |
Estimated Fair Value | 1,187 |
2042 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3,421 |
Estimated Fair Value | 3,443 |
2043 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 6,121 |
Estimated Fair Value | 6,394 |
2044 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 7,826 |
Estimated Fair Value | 8,164 |
2045 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 7,143 |
Estimated Fair Value | 7,418 |
2046 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 10,898 |
Estimated Fair Value | 11,170 |
2047 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 13,598 |
Estimated Fair Value | 13,935 |
2048 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,966 |
Estimated Fair Value | 3,070 |
2049 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 24,506 |
Estimated Fair Value | 25,449 |
2050 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 148,550 |
Estimated Fair Value | 145,645 |
2051 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 149,307 |
Estimated Fair Value | 146,869 |
2053 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 148,730 |
Estimated Fair Value | 149,218 |
2054 |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 56,247 |
Estimated Fair Value | $ 56,881 |

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