Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Available-for-sale securities, investments:
Amortized cost	$ 855,141
Gross unrealized gains	64,081
Gross unrealized losses more than 12 months	(12,760)
Gross unrealized losses less than 12 months	(2,016)
Estimated market value	904,446	766,792
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	28,023
Gross unrealized gains	2,916
Gross unrealized losses more than 12 months	(4)
Gross unrealized losses less than 12 months	(17)
Estimated market value	30,918
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	43,763
Gross unrealized gains	4,512
Gross unrealized losses less than 12 months	(8)
Estimated market value	48,267
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	147,735
Gross unrealized gains	14,657
Gross unrealized losses more than 12 months	(2)
Gross unrealized losses less than 12 months	(181)
Estimated market value	162,209
Corporate securities
Available-for-sale securities, investments:
Amortized cost	565,467
Gross unrealized gains	40,306
Gross unrealized losses more than 12 months	(1,042)
Gross unrealized losses less than 12 months	(1,720)
Estimated market value	603,011
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	18,244
Gross unrealized gains	431
Gross unrealized losses more than 12 months	(21)
Estimated market value	18,654
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	24,147
Gross unrealized gains	1,190
Gross unrealized losses more than 12 months	(689)
Gross unrealized losses less than 12 months	(90)
Estimated market value	24,558
Common stock
Available-for-sale securities, investments:
Amortized cost	27,762
Gross unrealized gains	69
Gross unrealized losses more than 12 months	(11,002)
Estimated market value	$ 16,829