Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Mar. 31, 2021
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,664,037	$ 2,405,612
Available-for-sale securities, fair value:
Estimated market value	2,821,092	2,653,539
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	97,969	90,142
Due after one year through five years	541,840	562,442
Due after five years through ten years	704,295	672,733
Due after ten years	995,904	905,567
Amortized cost	2,340,008	2,230,884
Available-for-sale securities, fair value:
Due in one year or less	99,432	91,190
Due after one year through five years	570,135	601,818
Due after five years through ten years	765,073	754,536
Due after ten years	1,056,295	1,019,570
Estimated market value	2,490,935	2,467,114
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	324,029	174,728
Available-for-sale securities, fair value:
Estimated market value	$ 330,157	$ 186,425

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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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