Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Available-for-sale securities, amortized cost:
Amortized cost	$ 1,135,544	$ 1,027,985
Available-for-sale securities, fair value:
Estimated market value	1,138,275	1,095,338
US Government and Corporate Securities [Member]
Available-for-sale securities, amortized cost:
Due in one year or less	20,235	43,658
Due after one year through five years	185,447	180,448
Due after five years through ten years	350,048	281,546
Due after ten years	531,645	448,169
Amortized cost	1,087,375	953,821
Available-for-sale securities, fair value:
Due in one year or less	20,475	44,333
Due after one year through five years	194,563	191,930
Due after five years through ten years	350,953	302,188
Due after ten years	521,289	486,780
Estimated market value	1,087,280	1,025,231
Mortgage backed securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	12,425	6,730
Available-for-sale securities, fair value:
Estimated market value	12,187	6,985
Redeemable Preferred Stocks [Member]
Available-for-sale securities, amortized cost:
Amortized cost	18,445	23,705
Available-for-sale securities, fair value:
Estimated market value	17,533	24,212
Equity Securities [Member]
Available-for-sale securities, amortized cost:
Amortized cost	17,299	43,729
Available-for-sale securities, fair value:
Estimated market value	$ 21,275	$ 38,910