Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Mar. 31, 2022
Available-for-sale securities, amortized cost:
Amortized cost	$ 3,006,587	$ 2,664,037
Available-for-sale securities, fair value:
Fair Value	2,709,037	2,821,092
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	354,875	97,969
Due after one year through five years	754,175	541,840
Due after five years through ten years	736,089	704,295
Due after ten years	790,568	995,904
Amortized cost	2,635,707	2,340,008
Available-for-sale securities, fair value:
Due in one year or less	354,184	99,432
Due after one year through five years	717,552	570,135
Due after five years through ten years	665,708	765,073
Due after ten years	654,201	1,056,295
Fair Value	2,391,645	2,490,935
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	370,880	324,029
Available-for-sale securities, fair value:
Fair Value	$ 317,392	$ 330,157

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