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Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Mar. 31, 2022
--- | --- | ---
Available-for-sale securities, amortized cost: |  |
Amortized cost | $ 2,989,120 | $ 2,664,037
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | 2,663,528 | 2,821,092
US Treasury Securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Due in one year or less | 333,984 | 97,969
Due after one year through five years | 602,861 | 541,840
Due after five years through ten years | 756,691 | 704,295
Due after ten years | 919,765 | 995,904
Amortized cost | 2,613,301 | 2,340,008
Available-for-sale securities, fair value: |  |
Due in one year or less | 333,803 | 99,432
Due after one year through five years | 575,794 | 570,135
Due after five years through ten years | 674,055 | 765,073
Due after ten years | 746,989 | 1,056,295
Available for sale investments, estimated market value | 2,330,641 | 2,490,935
Mortgage-backed securities [Member] |  |
Available-for-sale securities, amortized cost: |  |
Amortized cost | 375,819 | 324,029
Available-for-sale securities, fair value: |  |
Available for sale investments, estimated market value | $ 332,887 | $ 330,157

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