Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Mar. 31, 2022
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,747,833	$ 2,664,037
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	2,552,311	2,821,092
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	114,799	97,969
Due after one year through five years	589,259	541,840
Due after five years through ten years	754,434	704,295
Due after ten years	924,687	995,904
Amortized cost	2,383,179	2,340,008
Available-for-sale securities, fair value:
Due in one year or less	114,987	99,432
Due after one year through five years	580,364	570,135
Due after five years through ten years	709,509	765,073
Due after ten years	813,061	1,056,295
Available for sale investments, estimated market value	2,217,921	2,490,935
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	364,654	324,029
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 334,390	$ 330,157

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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