Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,847,422	$ 1,630,600
Available-for-sale securities, fair value:
Estimated market value	1,919,860	1,663,768
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	36,446	35,399
Due after one year through five years	441,223	324,286
Due after five years through ten years	607,895	598,232
Due after ten years	641,411	616,585
Amortized cost	1,726,975	1,574,502
Available-for-sale securities, fair value:
Due in one year or less	36,674	35,795
Due after one year through five years	450,816	333,016
Due after five years through ten years	626,174	607,184
Due after ten years	671,448	622,763
Estimated market value	1,785,112	1,598,758
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	94,106	26,577
Available-for-sale securities, fair value:
Estimated market value	96,025	27,087
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	10,609	13,789
Available-for-sale securities, fair value:
Estimated market value	10,861	13,489
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,732	15,732
Available-for-sale securities, fair value:
Estimated market value	$ 27,862	$ 24,434