Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,558,279	$ 2,405,612
Available-for-sale securities, fair value:
Estimated market value	2,703,052	2,653,539
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	57,753	90,142
Due after one year through five years	590,448	562,442
Due after five years through ten years	641,992	672,733
Due after ten years	1,097,517	905,567
Amortized cost	2,387,710	2,230,884
Available-for-sale securities, fair value:
Due in one year or less	58,374	91,190
Due after one year through five years	627,370	601,818
Due after five years through ten years	694,439	754,536
Due after ten years	1,141,923	1,019,570
Estimated market value	2,522,106	2,467,114
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	170,569	174,728
Available-for-sale securities, fair value:
Estimated market value	180,946	186,425
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	0	0
Available-for-sale securities, fair value:
Estimated market value	$ 0	$ 0

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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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