Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,318,588	$ 1,237,386
Available-for-sale securities, fair value:
Estimated market value	1,360,780	1,304,962
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	53,310	36,355
Due after one year through five years	185,138	198,488
Due after five years through ten years	502,261	474,639
Due after ten years	522,263	472,003
Amortized cost	1,262,972	1,181,485
Available-for-sale securities, fair value:
Due in one year or less	54,085	37,055
Due after one year through five years	195,011	209,404
Due after five years through ten years	514,431	492,782
Due after ten years	534,855	502,092
Estimated market value	1,298,382	1,241,333
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	19,079	19,874
Available-for-sale securities, fair value:
Estimated market value	19,699	20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,055	18,052
Available-for-sale securities, fair value:
Estimated market value	18,098	18,296
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,482	17,975
Available-for-sale securities, fair value:
Estimated market value	$ 24,601	$ 24,654