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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Mar. 31, 2024
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,631,881 | $ 2,660,093
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | 2,395,890 | 2,442,504
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 301,514 | 266,357
Due after one year through five years | 608,709 | 748,338
Due after five years through ten years | 664,240 | 614,890
Due after ten years | 758,127 | 723,998
Amortized cost | 2,332,590 | 2,353,583
Available-for-sale securities, fair value: |  |
Due in one year or less | 299,952 | 266,578
Due after one year through five years | 587,286 | 723,903
Due after five years through ten years | 605,525 | 564,422
Due after ten years | 648,323 | 626,764
Available for sale investments, estimated market value | 2,141,086 | 2,181,667
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 299,291 | 306,510
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | $ 254,804 | $ 260,837

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