Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Available-for-sale securities, investments:
Amortized cost	$ 754,941
Gross unrealized gains	53,144
Gross unrealized losses more than 12 months	(12,822)
Gross unrealized losses less than 12 months	(1,881)
Estimated market value	793,382	766,792
U.S. treasury, government, and corporate securities
Available-for-sale securities, investments:
Amortized cost	694,950
Gross unrealized gains	51,673
Gross unrealized losses more than 12 months	(315)
Gross unrealized losses less than 12 months	(1,832)
Estimated market value	744,476
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	28,722
Gross unrealized gains	2,510
Gross unrealized losses more than 12 months	(4)
Gross unrealized losses less than 12 months	(8)
Estimated market value	31,220
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	46,507
Gross unrealized gains	4,615
Gross unrealized losses more than 12 months	0
Gross unrealized losses less than 12 months	(7)
Estimated market value	51,115
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	146,121
Gross unrealized gains	11,481
Gross unrealized losses more than 12 months	0
Gross unrealized losses less than 12 months	(151)
Estimated market value	157,451
Corporate securities
Available-for-sale securities, investments:
Amortized cost	473,600
Gross unrealized gains	33,067
Gross unrealized losses more than 12 months	(311)
Gross unrealized losses less than 12 months	(1,666)
Estimated market value	504,690
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	8,885
Gross unrealized gains	269
Gross unrealized losses more than 12 months	(15)
Gross unrealized losses less than 12 months	(1)
Estimated market value	9,138
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	23,370
Gross unrealized gains	1,155
Gross unrealized losses more than 12 months	(547)
Gross unrealized losses less than 12 months	(43)
Estimated market value	23,935
Common stock
Available-for-sale securities, investments:
Amortized cost	27,736
Gross unrealized gains	47
Gross unrealized losses more than 12 months	(11,945)
Gross unrealized losses less than 12 months	(5)
Estimated market value	$ 15,833