Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Available-for-sale securities, investments:
Amortized cost	$ 1,135,544	$ 1,027,985
Gross unrealized gains	39,491	75,734
Gross unrealized losses more than 12 months	(4,192)	(5,311)
Gross unrealized losses less than 12 months	(32,568)	(3,070)
Estimated market value	1,138,275	1,095,338
US Government and Corporate Securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,087,375	953,821
Estimated market value	1,087,280	1,025,231
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	49,883	24,518
Gross unrealized gains	1,475	2,749
Gross unrealized losses more than 12 months		(3)
Gross unrealized losses less than 12 months	(1,004)	(3)
Estimated market value	50,354	27,261
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	36,258	44,934
Gross unrealized gains	2,558	4,027
Gross unrealized losses more than 12 months	(4)	(7)
Gross unrealized losses less than 12 months	(425)	(73)
Estimated market value	38,387	48,881
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	166,311	154,692
Gross unrealized gains	4,834	16,056
Gross unrealized losses more than 12 months	(308)	(2)
Gross unrealized losses less than 12 months	(3,627)	(292)
Estimated market value	167,210	170,454
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	834,923	729,677
Gross unrealized gains	26,075	51,596
Gross unrealized losses more than 12 months	(3,794)	(373)
Gross unrealized losses less than 12 months	(25,875)	(2,265)
Estimated market value	831,329	778,635
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	12,425	6,730
Gross unrealized gains	279	282
Gross unrealized losses more than 12 months	(3)	(27)
Gross unrealized losses less than 12 months	(514)
Estimated market value	12,187	6,985
Redeemable Preferred Stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,445	23,705
Gross unrealized gains	283	751
Gross unrealized losses more than 12 months	(82)	(214)
Gross unrealized losses less than 12 months	(1,113)	(30)
Estimated market value	17,533	24,212
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,299	43,729
Gross unrealized gains	3,987	273
Gross unrealized losses more than 12 months	(1)	(4,685)
Gross unrealized losses less than 12 months	(10)	(407)
Estimated market value	$ 21,275	$ 38,910