Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Mar. 31, 2023
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,816,481	$ 3,006,587
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	2,474,624	2,709,037
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	323,343	354,875
Due after one year through five years	744,486	754,175
Due after five years through ten years	650,412	736,089
Due after ten years	767,738	790,568
Amortized cost	2,485,979	2,635,707
Available-for-sale securities, fair value:
Due in one year or less	323,638	354,184
Due after one year through five years	705,900	717,552
Due after five years through ten years	566,624	665,708
Due after ten years	606,440	654,201
Available for sale investments, estimated market value	2,202,602	2,391,645
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	330,502	370,880
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 272,022	$ 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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