Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,263,299	$ 2,334,215
Available-for-sale securities, fair value:
Estimated market value	2,459,159	2,467,613
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	112,348	128,747
Due after one year through five years	532,931	547,821
Due after five years through ten years	626,777	636,036
Due after ten years	790,921	832,334
Amortized cost	2,062,977	2,144,938
Available-for-sale securities, fair value:
Due in one year or less	110,836	129,420
Due after one year through five years	562,558	566,934
Due after five years through ten years	691,999	678,636
Due after ten years	882,203	897,371
Estimated market value	2,247,596	2,272,361
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	198,829	187,784
Available-for-sale securities, fair value:
Estimated market value	210,033	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	$ 1,530	$ 1,565

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