Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2012
Available-for-sale securities, investments:
Amortized cost	$ 1,027,985	$ 730,025
Gross unrealized gains	75,734	52,000
Gross unrealized losses more than 12 months	(5,311)	(12,023)
Gross unrealized losses less than 12 months	(3,070)	(3,210)
Estimated market value	1,095,338	766,792
U.S. treasury, government, and corporate securities
Available-for-sale securities, investments:
Amortized cost	953,821	666,347
Gross unrealized gains	74,428	50,615
Gross unrealized losses more than 12 months	(385)	(638)
Gross unrealized losses less than 12 months	(2,633)	(2,399)
Estimated market value	1,025,231	713,925
U.S. treasury securities and government obligation
Available-for-sale securities, investments:
Amortized cost	24,518	29,152
Gross unrealized gains	2,749	2,964
Gross unrealized losses more than 12 months	(3)	(18)
Gross unrealized losses less than 12 months	(3)	(9)
Estimated market value	27,261	32,089
U.S. government agency mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	44,934	48,938
Gross unrealized gains	4,027	4,866
Gross unrealized losses more than 12 months	(7)	(1)
Gross unrealized losses less than 12 months	(73)	(7)
Estimated market value	48,881	53,796
Obligations of states and political subdivisions
Available-for-sale securities, investments:
Amortized cost	154,692	142,824
Gross unrealized gains	16,056	9,435
Gross unrealized losses more than 12 months	(2)	0
Gross unrealized losses less than 12 months	(292)	(147)
Estimated market value	170,454	152,112
Corporate securities
Available-for-sale securities, investments:
Amortized cost	729,677	445,433
Gross unrealized gains	51,596	33,350
Gross unrealized losses more than 12 months	(373)	(619)
Gross unrealized losses less than 12 months	(2,265)	(2,236)
Estimated market value	778,635	475,928
Mortgage-backed securities
Available-for-sale securities, investments:
Amortized cost	6,730	11,572
Gross unrealized gains	282	282
Gross unrealized losses more than 12 months	(27)	(38)
Gross unrealized losses less than 12 months	0	(5)
Estimated market value	6,985	11,811
Redeemable preferred stock
Available-for-sale securities, investments:
Amortized cost	23,705	24,370
Gross unrealized gains	751	1,066
Gross unrealized losses more than 12 months	(214)	(1,627)
Gross unrealized losses less than 12 months	(30)	(632)
Estimated market value	24,212	23,177
Common Stock
Available-for-sale securities, investments:
Amortized cost	43,729	27,736
Gross unrealized gains	273	37
Gross unrealized losses more than 12 months	(4,685)	(9,720)
Gross unrealized losses less than 12 months	(407)	(174)
Estimated market value	$ 38,910	$ 17,879