Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Mar. 31, 2021
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,586,674	$ 2,405,612
Available-for-sale securities, fair value:
Estimated market value	2,790,243	2,653,539
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	48,717	90,142
Due after one year through five years	575,511	562,442
Due after five years through ten years	693,842	672,733
Due after ten years	1,095,998	905,567
Amortized cost	2,414,068	2,230,884
Available-for-sale securities, fair value:
Due in one year or less	49,480	91,190
Due after one year through five years	613,941	601,818
Due after five years through ten years	768,285	754,536
Due after ten years	1,174,649	1,019,570
Estimated market value	2,606,355	2,467,114
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	172,606	174,728
Available-for-sale securities, fair value:
Estimated market value	$ 183,888	$ 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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