Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Mar. 31, 2014
Available-for-sale securities, investments:
Amortized cost	$ 1,179,977	$ 1,135,544
Gross unrealized gains	51,825	39,491
Gross unrealized losses more than 12 months	(5,983)	(4,192)
Gross unrealized losses less than 12 months	(12,507)	(32,568)
Estimated market value	1,213,312	1,138,275
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	48,959	49,883
Gross unrealized gains	2,405	1,475
Gross unrealized losses more than 12 months	(261)
Gross unrealized losses less than 12 months	(83)	(1,004)
Estimated market value	51,020	50,354
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	34,679	36,258
Gross unrealized gains	2,754	2,558
Gross unrealized losses more than 12 months	(332)	(4)
Gross unrealized losses less than 12 months	(18)	(425)
Estimated market value	37,083	38,387
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	167,039	166,311
Gross unrealized gains	7,955	4,834
Gross unrealized losses more than 12 months	(358)	(308)
Gross unrealized losses less than 12 months	(1,607)	(3,627)
Estimated market value	173,029	167,210
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	877,948	834,923
Gross unrealized gains	34,019	26,075
Gross unrealized losses more than 12 months	(5,028)	(3,794)
Gross unrealized losses less than 12 months	(9,838)	(25,875)
Estimated market value	897,101	831,329
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	15,335	12,425
Gross unrealized gains	296	279
Gross unrealized losses more than 12 months	(2)	(3)
Gross unrealized losses less than 12 months	(272)	(514)
Estimated market value	15,357	12,187
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,447	18,445
Gross unrealized gains	426	283
Gross unrealized losses more than 12 months		(82)
Gross unrealized losses less than 12 months	(669)	(1,113)
Estimated market value	17,204	17,533
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,570	17,299
Gross unrealized gains	3,970	3,987
Gross unrealized losses more than 12 months	(2)	(1)
Gross unrealized losses less than 12 months	(20)	(10)
Estimated market value	$ 22,518	$ 21,275