Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Available-for-sale securities, amortized cost:
Due in one year or less	$ 51,141
Due after one year through five years	148,038
Due after five years through ten years	217,112
Due after ten years	368,697
Amortized cost	855,141
Available-for-sale securities, fair value:
Due in one year or less	52,289
Due after one year through five years	157,500
Due after five years through ten years	233,269
Due after ten years	401,347
Estimated market value	904,446	766,792
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	18,244
Available-for-sale securities, fair value:
Estimated market value	18,654
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	24,147
Available-for-sale securities, fair value:
Estimated market value	24,558
Common Stock [Member]
Available-for-sale securities, amortized cost:
Amortized cost	27,762
Available-for-sale securities, fair value:
Estimated market value	$ 16,829