Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Mar. 31, 2019
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,277,468	$ 2,228,961
Available-for-sale securities, fair value:
Estimated market value	2,386,687	2,211,229
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	83,028	71,987
Due after one year through five years	521,038	541,195
Due after five years through ten years	694,348	621,031
Due after ten years	827,418	845,052
Amortized cost	2,125,832	2,079,265
Available-for-sale securities, fair value:
Due in one year or less	83,337	71,954
Due after one year through five years	536,225	540,658
Due after five years through ten years	729,026	614,485
Due after ten years	880,910	834,769
Estimated market value	2,229,498	2,061,866
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	150,143	148,203
Available-for-sale securities, fair value:
Estimated market value	155,622	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	$ 1,567	$ 1,468