Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,299,673	$ 2,228,961
Available-for-sale securities, fair value:
Estimated market value	2,441,979	2,211,229
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	114,557	71,987
Due after one year through five years	554,591	541,195
Due after five years through ten years	662,534	621,031
Due after ten years	812,618	845,052
Amortized cost	2,144,300	2,079,265
Available-for-sale securities, fair value:
Due in one year or less	115,001	71,954
Due after one year through five years	572,647	540,658
Due after five years through ten years	707,015	614,485
Due after ten years	884,262	834,769
Estimated market value	2,278,925	2,061,866
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	153,880	148,203
Available-for-sale securities, fair value:
Estimated market value	161,479	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	$ 1,575	$ 1,468