Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Mar. 31, 2024
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,776,940	$ 2,660,093
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	2,617,126	2,442,504
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	211,754	266,357
Due after one year through five years	632,864	748,338
Due after five years through ten years	602,819	614,890
Due after ten years	799,768	723,998
Amortized cost	2,247,205	2,353,583
Available-for-sale securities, fair value:
Due in one year or less	210,698	266,578
Due after one year through five years	626,490	723,903
Due after five years through ten years	568,543	564,422
Due after ten years	714,230	626,764
Available for sale investments, estimated market value	2,119,961	2,181,667
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	529,735	306,510
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 497,165	$ 260,837

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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