Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,405,612	$ 2,334,215
Available-for-sale securities, fair value:
Estimated market value	2,653,539	2,467,613
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	90,142	128,747
Due after one year through five years	562,442	547,821
Due after five years through ten years	672,733	636,036
Due after ten years	905,567	832,334
Amortized cost	2,230,884	2,144,938
Available-for-sale securities, fair value:
Due in one year or less	91,190	129,420
Due after one year through five years	601,818	566,934
Due after five years through ten years	754,536	678,636
Due after ten years	1,019,570	897,371
Estimated market value	2,467,114	2,272,361
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	174,728	187,784
Available-for-sale securities, fair value:
Estimated market value	186,425	193,687
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	0	1,493
Available-for-sale securities, fair value:
Estimated market value	$ 0	$ 1,565