Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,668,172	$ 3,006,587
Available-for-sale securities, fair value:
Fair Value	2,407,226	2,709,037
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	134,115	354,875
Due after one year through five years	734,777	754,175
Due after five years through ten years	699,066	736,089
Due after ten years	740,801	790,568
Amortized cost	2,308,759	2,635,707
Available-for-sale securities, fair value:
Due in one year or less	130,997	354,184
Due after one year through five years	698,857	717,552
Due after five years through ten years	637,939	665,708
Due after ten years	631,066	654,201
Fair Value	2,098,859	2,391,645
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	359,413	370,880
Available-for-sale securities, fair value:
Fair Value	$ 308,367	$ 317,392

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