Investments (Adjusted cost and estimated market value of available-for-sale investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,253,575	$ 1,237,386
Available-for-sale securities, fair value:
Estimated market value	1,334,199	1,304,962
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	52,734	36,355
Due after one year through five years	182,786	198,488
Due after five years through ten years	494,677	474,639
Due after ten years	467,923	472,003
Amortized cost	1,198,120	1,181,485
Available-for-sale securities, fair value:
Due in one year or less	53,626	37,055
Due after one year through five years	194,033	209,404
Due after five years through ten years	519,766	492,782
Due after ten years	503,153	502,092
Estimated market value	1,270,578	1,241,333
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	19,259	19,874
Available-for-sale securities, fair value:
Estimated market value	20,252	20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,053	18,052
Available-for-sale securities, fair value:
Estimated market value	18,552	18,296
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,143	17,975
Available-for-sale securities, fair value:
Estimated market value	$ 24,817	$ 24,654