Adjusted Cost and Estimated Market Value Available-for-sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Available-for-sale securities, amortized cost:
Amortized cost	$ 754,941
Available-for-sale securities, fair value:
Estimated market value	793,382	766,792
U.S. treasury, government, and corporate securities
Available-for-sale securities, amortized cost:
Due in one year or less	43,244
Due after one year through five years	151,997
Due after five years through ten years	187,143
Due after ten years	312,566
Amortized cost	694,950
Available-for-sale securities, fair value:
Due in one year or less	43,995
Due after one year through five years	161,472
Due after five years through ten years	200,440
Due after ten years	338,569
Estimated market value	744,476
Mortgage-backed securities
Available-for-sale securities, amortized cost:
Amortized cost	8,885
Available-for-sale securities, fair value:
Estimated market value	9,138
Redeemable preferred stock
Available-for-sale securities, amortized cost:
Amortized cost	23,370
Available-for-sale securities, fair value:
Estimated market value	23,935
Equity security investments
Available-for-sale securities, amortized cost:
Amortized cost	27,736
Available-for-sale securities, fair value:
Estimated market value	$ 15,833