Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Mar. 31, 2023
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,758,446	$ 3,006,587
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	2,472,128	2,709,037
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	251,106	354,875
Due after one year through five years	599,626	754,175
Due after five years through ten years	659,400	736,089
Due after ten years	733,486	790,568
Amortized cost	2,243,618	2,635,707
Available-for-sale securities, fair value:
Due in one year or less	249,366	354,184
Due after one year through five years	569,012	717,552
Due after five years through ten years	590,567	665,708
Due after ten years	612,788	654,201
Available for sale investments, estimated market value	2,021,733	2,391,645
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	514,679	370,880
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 450,248	$ 317,392

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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