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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Mar. 31, 2023
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,660,093 | $ 3,006,587
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | 2,442,504 | 2,709,037
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 266,357 | 354,875
Due after one year through five years | 748,338 | 754,175
Due after five years through ten years | 614,890 | 736,089
Due after ten years | 723,998 | 790,568
Amortized cost | 2,353,583 | 2,635,707
Available-for-sale securities, fair value: |  |
Due in one year or less | 266,578 | 354,184
Due after one year through five years | 723,903 | 717,552
Due after five years through ten years | 564,422 | 665,708
Due after ten years | 626,764 | 654,201
Available for sale investments, estimated market value | 2,181,667 | 2,391,645
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 306,510 | 370,880
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | $ 260,837 | $ 317,392

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