Available-for-Sale Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Mar. 31, 2014
Available-for-sale securities, investments:
Amortized cost	$ 1,200,238	$ 1,135,544
Gross unrealized gains	67,573	39,491
Gross unrealized losses more than 12 months	(6,238)	(4,192)
Gross unrealized losses less than 12 months	(1,442)	(32,568)
Estimated market value	1,260,131	1,138,275
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	71,987	49,883
Gross unrealized gains	3,640	1,475
Gross unrealized losses more than 12 months	(223)	0
Gross unrealized losses less than 12 months	(20)	(1,004)
Estimated market value	75,384	50,354
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	33,291	36,258
Gross unrealized gains	2,838	2,558
Gross unrealized losses more than 12 months	(129)	(4)
Gross unrealized losses less than 12 months	(3)	(425)
Estimated market value	35,997	38,387
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	164,887	166,311
Gross unrealized gains	9,354	4,834
Gross unrealized losses more than 12 months	(1,242)	(308)
Gross unrealized losses less than 12 months	(39)	(3,627)
Estimated market value	172,960	167,210
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	877,833	834,923
Gross unrealized gains	45,736	26,075
Gross unrealized losses more than 12 months	(4,294)	(3,794)
Gross unrealized losses less than 12 months	(1,275)	(25,875)
Estimated market value	918,000	831,329
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	16,817	12,425
Gross unrealized gains	518	279
Gross unrealized losses more than 12 months	(2)	(3)
Gross unrealized losses less than 12 months	(80)	(514)
Estimated market value	17,253	12,187
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,448	18,445
Gross unrealized gains	646	283
Gross unrealized losses more than 12 months	(348)	(82)
Gross unrealized losses less than 12 months	(25)	(1,113)
Estimated market value	17,721	17,533
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,975	17,299
Gross unrealized gains	4,841	3,987
Gross unrealized losses more than 12 months	0	(1)
Gross unrealized losses less than 12 months	0	(10)
Estimated market value	$ 22,816	$ 21,275