Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013
Available-for-sale securities, investments:
Amortized cost	$ 1,050,222
Gross unrealized gains	75,195
Gross unrealized losses more than 12 months	(3,664)
Gross unrealized losses less than 12 months	(4,745)
Estimated market value	1,117,008	1,095,338
U.S. treasury, government, and corporate securities
Available-for-sale securities, investments:
Amortized cost	979,723
Gross unrealized gains	72,413
Gross unrealized losses more than 12 months	(203)
Gross unrealized losses less than 12 months	(4,695)
Estimated market value	1,047,238
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	28,083
Gross unrealized gains	2,568
Gross unrealized losses more than 12 months	(3)
Gross unrealized losses less than 12 months	(3)
Estimated market value	30,645
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	42,332
Gross unrealized gains	3,627
Gross unrealized losses more than 12 months	(5)
Gross unrealized losses less than 12 months	(34)
Estimated market value	45,920
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	154,644
Gross unrealized gains	16,482
Gross unrealized losses more than 12 months	0
Gross unrealized losses less than 12 months	(382)
Estimated market value	170,744
Corporate securities
Available-for-sale securities, investments:
Amortized cost	754,664
Gross unrealized gains	49,736
Gross unrealized losses more than 12 months	(195)
Gross unrealized losses less than 12 months	(4,276)
Estimated market value	799,929
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	6,117
Gross unrealized gains	251
Gross unrealized losses more than 12 months	(28)
Gross unrealized losses less than 12 months	0
Estimated market value	6,340
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	21,163
Gross unrealized gains	871
Gross unrealized losses more than 12 months	(28)
Gross unrealized losses less than 12 months	(6)
Estimated market value	22,000
Common stock
Available-for-sale securities, investments:
Amortized cost	43,219
Gross unrealized gains	1,660
Gross unrealized losses more than 12 months	(3,405)
Gross unrealized losses less than 12 months	(44)
Estimated market value	$ 41,430