Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Mar. 31, 2020
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,351,246	$ 2,334,215
Available-for-sale securities, fair value:
Estimated market value	2,566,813	2,467,613
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	113,929	128,747
Due after one year through five years	555,675	547,821
Due after five years through ten years	656,915	636,036
Due after ten years	820,481	832,334
Amortized cost	2,147,000	2,144,938
Available-for-sale securities, fair value:
Due in one year or less	114,681	129,420
Due after one year through five years	590,071	566,934
Due after five years through ten years	726,982	678,636
Due after ten years	917,120	897,371
Estimated market value	2,348,854	2,272,361
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	202,753	187,784
Available-for-sale securities, fair value:
Estimated market value	216,439	193,687
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	1,493	1,493
Available-for-sale securities, fair value:
Estimated market value	$ 1,520	$ 1,565

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