Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Mar. 31, 2022
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,989,120	$ 2,664,037
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	2,663,528	2,821,092
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	333,984	97,969
Due after one year through five years	602,861	541,840
Due after five years through ten years	756,691	704,295
Due after ten years	919,765	995,904
Amortized cost	2,613,301	2,340,008
Available-for-sale securities, fair value:
Due in one year or less	333,803	99,432
Due after one year through five years	575,794	570,135
Due after five years through ten years	674,055	765,073
Due after ten years	746,989	1,056,295
Available for sale investments, estimated market value	2,330,641	2,490,935
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	375,819	324,029
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 332,887	$ 330,157

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Amount 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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