Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Available-for-sale securities, investments:
Amortized cost	$ 950,758
Gross unrealized gains	77,558
Gross unrealized losses more than 12 months	(7,121)
Gross unrealized losses less than 12 months	(1,227)
Estimated market value	1,019,968	766,792
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	21,201
Gross unrealized gains	2,910
Gross unrealized losses more than 12 months	(3)
Gross unrealized losses less than 12 months	0
Estimated market value	24,108
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	41,010
Gross unrealized gains	4,427
Gross unrealized losses more than 12 months	(7)
Gross unrealized losses less than 12 months	(1)
Estimated market value	45,429
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	149,644
Gross unrealized gains	16,073
Gross unrealized losses more than 12 months	(2)
Gross unrealized losses less than 12 months	(54)
Estimated market value	165,661
Corporate securities
Available-for-sale securities, investments:
Amortized cost	662,087
Gross unrealized gains	52,067
Gross unrealized losses more than 12 months	(650)
Gross unrealized losses less than 12 months	(1,131)
Estimated market value	712,373
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	24,597
Gross unrealized gains	774
Gross unrealized losses more than 12 months	(25)
Gross unrealized losses less than 12 months	(30)
Estimated market value	25,316
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	21,353
Gross unrealized gains	1,208
Gross unrealized losses more than 12 months	(342)
Gross unrealized losses less than 12 months	(11)
Estimated market value	22,208
Common stock
Available-for-sale securities, investments:
Amortized cost	30,866
Gross unrealized gains	99
Gross unrealized losses more than 12 months	(6,092)
Gross unrealized losses less than 12 months	0
Estimated market value	$ 24,873