Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
Available-for-sale securities, amortized cost:
Amortized cost	$ 2,228,961	$ 1,823,199
Available-for-sale securities, fair value:
Estimated market value	2,211,229	1,883,384
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	71,987	36,446
Due after one year through five years	541,195	441,159
Due after five years through ten years	621,031	607,075
Due after ten years	845,052	621,820
Amortized cost	2,079,265	1,706,500
Available-for-sale securities, fair value:
Due in one year or less	71,954	36,674
Due after one year through five years	540,658	450,749
Due after five years through ten years	614,485	625,282
Due after ten years	834,769	651,554
Estimated market value	2,061,866	1,764,259
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	148,203	114,581
Available-for-sale securities, fair value:
Estimated market value	147,895	116,878
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	1,493	2,118
Available-for-sale securities, fair value:
Estimated market value	$ 1,468	$ 2,247