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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Mar. 31, 2024
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,682,830 | $ 2,660,093
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | 2,437,923 | 2,442,504
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 274,397 | 266,357
Due after one year through five years | 645,371 | 748,338
Due after five years through ten years | 614,127 | 614,890
Due after ten years | 746,656 | 723,998
Amortized cost | 2,280,551 | 2,353,583
Available-for-sale securities, fair value: |  |
Due in one year or less | 272,854 | 266,578
Due after one year through five years | 623,920 | 723,903
Due after five years through ten years | 554,516 | 564,422
Due after ten years | 629,921 | 626,764
Available for sale investments, estimated market value | 2,081,211 | 2,181,667
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 402,279 | 306,510
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | $ 356,712 | $ 260,837

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