Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,903,719	$ 1,823,199
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	1,916,049	1,883,384
US Treasury Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	42,372	36,446
Due after one year through five years	469,929	441,223
Due after five years through ten years	616,785	607,895
Due after ten years	670,910	641,411
Amortized cost by contractural maturity	1,799,996	1,726,975
Available-for-sale securities, fair value:
Due in one year or less	42,546	36,674
Due after one year through five years	472,652	450,816
Due after five years through ten years	618,194	626,174
Due after ten years	678,551	671,448
Estimated market value by contractural maturity	1,811,943	1,785,112
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	101,605	94,106
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	101,895	96,025
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	2,118	2,118
Available-for-sale securities, fair value:
Available for sale investments, estimated market value	$ 2,211	$ 2,247