Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,334,215	$ 2,228,961
Available-for-sale securities, fair value:
Estimated market value	2,467,613	2,211,229
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	128,747	71,987
Due after one year through five years	547,821	541,195
Due after five years through ten years	636,036	621,031
Due after ten years	832,334	845,052
Amortized cost	2,144,938	2,079,265
Available-for-sale securities, fair value:
Due in one year or less	129,420	71,954
Due after one year through five years	566,934	540,658
Due after five years through ten years	678,636	614,485
Due after ten years	897,371	834,769
Estimated market value	2,272,361	2,061,866
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	187,784	148,203
Available-for-sale securities, fair value:
Estimated market value	193,687	147,895
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	$ 1,565	$ 1,468