Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Mar. 31, 2015
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,420,928	$ 1,237,386
Available-for-sale securities, fair value:
Estimated market value	1,463,997	1,304,962
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	51,630	36,355
Due after one year through five years	206,310	198,488
Due after five years through ten years	515,059	474,639
Due after ten years	568,305	472,003
Amortized cost	1,341,304	1,181,485
Available-for-sale securities, fair value:
Due in one year or less	52,331	37,055
Due after one year through five years	216,005	209,404
Due after five years through ten years	526,263	492,782
Due after ten years	583,038	502,092
Estimated market value	1,377,637	1,241,333
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	43,076	19,874
Available-for-sale securities, fair value:
Estimated market value	44,090	20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,056	18,052
Available-for-sale securities, fair value:
Estimated market value	18,170	18,296
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,492	17,975
Available-for-sale securities, fair value:
Estimated market value	$ 24,100	$ 24,654