Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Mar. 31, 2014
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,200,238	$ 1,135,544
Available-for-sale securities, fair value:
Estimated market value	1,260,131	1,138,275
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	40,037	20,235
Due after one year through five years	225,426	185,447
Due after five years through ten years	472,125	350,048
Due after ten years	410,410	531,645
Amortized cost	1,147,998	1,087,375
Available-for-sale securities, fair value:
Due in one year or less	41,254	20,475
Due after one year through five years	239,240	194,563
Due after five years through ten years	495,392	350,953
Due after ten years	426,455	521,289
Estimated market value	1,202,341	1,087,280
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	16,817	12,425
Available-for-sale securities, fair value:
Estimated market value	17,253	12,187
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,448	18,445
Available-for-sale securities, fair value:
Estimated market value	17,721	17,533
Common stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,975	17,299
Available-for-sale securities, fair value:
Estimated market value	$ 22,816	$ 21,275