Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,317,006	$ 2,334,215
Available-for-sale securities, fair value:
Estimated market value	2,372,323	2,467,613
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	141,789	128,747
Due after one year through five years	537,400	547,821
Due after five years through ten years	605,305	636,036
Due after ten years	833,420	832,334
Amortized cost	2,117,914	2,144,938
Available-for-sale securities, fair value:
Due in one year or less	141,448	129,420
Due after one year through five years	541,406	566,934
Due after five years through ten years	618,209	678,636
Due after ten years	875,707	897,371
Estimated market value	2,176,770	2,272,361
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	197,599	187,784
Available-for-sale securities, fair value:
Estimated market value	194,051	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	$ 1,502	$ 1,565

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