Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,640,198	$ 2,664,037
Available-for-sale securities, fair value:
Estimated market value	2,598,199	2,821,092
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	104,389	97,969
Due after one year through five years	554,348	541,840
Due after five years through ten years	713,435	704,295
Due after ten years	931,003	995,904
Amortized cost	2,303,175	2,340,008
Available-for-sale securities, fair value:
Due in one year or less	105,325	99,432
Due after one year through five years	557,915	570,135
Due after five years through ten years	717,393	765,073
Due after ten years	894,710	1,056,295
Estimated market value	2,275,343	2,490,935
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	337,023	324,029
Available-for-sale securities, fair value:
Estimated market value	$ 322,856	$ 330,157

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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 sixth through tenth 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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