Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Mar. 31, 2015
Available-for-sale securities, investments:
Amortized cost	$ 1,318,588	$ 1,237,386
Gross unrealized gains	57,080	73,796
Gross unrealized losses more than 12 months	(1,724)	(3,177)
Gross unrealized losses less than 12 months	(13,164)	(3,043)
Estimated market value	1,360,780	1,304,962
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	91,443	99,722
Gross unrealized gains	$ 3,681	5,658
Gross unrealized losses more than 12 months		$ (64)
Gross unrealized losses less than 12 months	$ (369)
Estimated market value	94,755	$ 105,316
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	28,050	30,569
Gross unrealized gains	2,397	2,614
Gross unrealized losses more than 12 months	(5)	(39)
Gross unrealized losses less than 12 months	(69)	(3)
Estimated market value	30,373	33,141
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	161,134	165,724
Gross unrealized gains	10,175	13,052
Gross unrealized losses more than 12 months	(82)	(298)
Gross unrealized losses less than 12 months	(607)	(10)
Estimated market value	170,620	178,468
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	982,345	885,470
Gross unrealized gains	33,646	44,426
Gross unrealized losses more than 12 months	(1,637)	(2,522)
Gross unrealized losses less than 12 months	(11,720)	(2,966)
Estimated market value	1,002,634	924,408
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	19,079	19,874
Gross unrealized gains	$ 666	806
Gross unrealized losses more than 12 months		$ (1)
Gross unrealized losses less than 12 months	$ (46)
Estimated market value	19,699	$ 20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,055	18,052
Gross unrealized gains	$ 351	521
Gross unrealized losses more than 12 months		(253)
Gross unrealized losses less than 12 months	$ (308)	(24)
Estimated market value	18,098	18,296
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,482	17,975
Gross unrealized gains	$ 6,164	$ 6,719
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	$ (45)	$ (40)
Estimated market value	$ 24,601	$ 24,654