Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014
Available-for-sale securities, amortized cost:
Available-for-sale investments, Amortized cost, total	$ 1,179,977	$ 1,135,544
Available-for-sale securities, fair value:
Available-for-sale investments, Estimated market value, total	1,213,312	1,138,275
Us Treasury, Government, and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	24,809	20,235
Due after one year through five years	205,496	185,447
Due after five years through ten years	484,926	350,048
Due after ten years	413,394	531,645
Available-for-sale investments, Amortized cost, subtotal	1,128,625	1,087,375
Available-for-sale securities, fair value:
Due in one year or less	25,355	20,475
Due after one year through five years	216,743	194,563
Due after five years through ten years	499,837	350,953
Due after ten years	416,298	521,289
Available-for-sale investments, Estimated market value, subtotal	1,158,233	1,087,280
Mortgage-backed securities [Member]
Available-for-sale securities, amortized cost:
Debt securities, amortized cost	15,335	12,425
Available-for-sale securities, fair value:
Debt securities, fair value	15,357	12,187
Redeemable preferred stocks [Member]
Available-for-sale securities, amortized cost:
Debt securities, amortized cost	17,447	18,445
Available-for-sale securities, fair value:
Debt securities, fair value	17,204	17,533
Common stocks [Member]
Available-for-sale securities, amortized cost:
Debt securities, amortized cost	18,570	17,299
Available-for-sale securities, fair value:
Debt securities, fair value	$ 22,518	$ 21,275