Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,496,256	$ 1,237,386
Available-for-sale securities, fair value:
Estimated market value	1,510,538	1,304,962
US Government and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	48,679	36,355
Due after one year through five years	250,576	198,488
Due after five years through ten years	557,984	474,639
Due after ten years	560,317	472,003
Amortized cost	1,417,556	1,181,485
Available-for-sale securities, fair value:
Due in one year or less	49,146	37,055
Due after one year through five years	256,597	209,404
Due after five years through ten years	557,961	492,782
Due after ten years	559,833	502,092
Estimated market value	1,423,537	1,241,333
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	42,991	19,874
Available-for-sale securities, fair value:
Estimated market value	43,404	20,679
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,977	18,052
Available-for-sale securities, fair value:
Estimated market value	18,428	18,296
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,732	17,975
Available-for-sale securities, fair value:
Estimated market value	$ 25,169	$ 24,654