Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Available-for-sale securities, amortized cost:
Amortized cost	$ 950,758
Available-for-sale securities, fair value:
Estimated market value	1,019,968	766,792
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	48,841
Due after one year through five years	168,987
Due after five years through ten years	243,324
Due after ten years	412,790
Available-for-sale securities, fair value:
Due in one year or less	49,768
Due after one year through five years	180,963
Due after five years through ten years	263,840
Due after ten years	453,000
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	24,597
Available-for-sale securities, fair value:
Estimated market value	25,316
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	21,353
Available-for-sale securities, fair value:
Estimated market value	22,208
Common stock
Available-for-sale securities, amortized cost:
Amortized cost	30,866
Available-for-sale securities, fair value:
Estimated market value	$ 24,873