Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,630,600	$ 1,476,507
Available-for-sale securities, fair value:
Estimated market value	1,663,768	1,490,789
Us Government Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	35,399	48,679
Due after one year through five years	324,286	250,576
Due after five years through ten years	598,232	557,984
Due after ten years	616,585	560,317
Amortized cost	1,574,502	1,417,556
Available-for-sale securities, fair value:
Due in one year or less	35,795	49,146
Due after one year through five years	333,016	256,597
Due after five years through ten years	607,184	557,961
Due after ten years	622,763	559,833
Estimated market value	1,598,758	1,423,537
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	26,577	24,991
Available-for-sale securities, fair value:
Estimated market value	27,087	25,404
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	13,789	17,977
Available-for-sale securities, fair value:
Estimated market value	13,489	18,428
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	15,732	15,983
Available-for-sale securities, fair value:
Estimated market value	$ 24,434	$ 23,420