Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Mar. 31, 2021
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,559,271	$ 2,405,612
Available-for-sale securities, fair value:
Estimated market value	2,743,151	2,653,539
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	69,891	90,142
Due after one year through five years	572,812	562,442
Due after five years through ten years	677,891	672,733
Due after ten years	1,075,247	905,567
Amortized cost	2,395,841	2,230,884
Available-for-sale securities, fair value:
Due in one year or less	71,172	91,190
Due after one year through five years	609,255	601,818
Due after five years through ten years	745,523	754,536
Due after ten years	1,143,769	1,019,570
Estimated market value	2,569,719	2,467,114
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	163,430	174,728
Available-for-sale securities, fair value:
Estimated market value	$ 173,432	$ 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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