Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015
Available-for-sale securities, investments:
Amortized cost	$ 1,253,575	$ 1,237,386
Gross unrealized gains	84,341	73,796
Gross unrealized losses more than 12 months	(1,578)	(3,177)
Gross unrealized losses less than 12 months	(2,139)	(3,043)
Estimated market value	1,334,199	1,304,962
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	88,561	99,722
Gross unrealized gains	$ 5,796	5,658
Gross unrealized losses more than 12 months		$ (64)
Gross unrealized losses less than 12 months	$ (33)
Estimated market value	94,324	$ 105,316
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	29,224	30,569
Gross unrealized gains	$ 2,712	2,614
Gross unrealized losses more than 12 months		(39)
Gross unrealized losses less than 12 months	$ (2)	(3)
Estimated market value	31,934	33,141
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	162,859	165,724
Gross unrealized gains	14,467	13,052
Gross unrealized losses more than 12 months	(34)	(298)
Gross unrealized losses less than 12 months	(87)	(10)
Estimated market value	177,205	178,468
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	917,476	885,470
Gross unrealized gains	52,995	44,426
Gross unrealized losses more than 12 months	(1,489)	(2,522)
Gross unrealized losses less than 12 months	(1,867)	(2,966)
Estimated market value	967,115	924,408
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	19,259	19,874
Gross unrealized gains	994	806
Gross unrealized losses more than 12 months	$ (1)	$ (1)
Gross unrealized losses less than 12 months
Estimated market value	$ 20,252	$ 20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,053	18,052
Gross unrealized gains	591	521
Gross unrealized losses more than 12 months	(54)	(253)
Gross unrealized losses less than 12 months	(38)	(24)
Estimated market value	18,552	18,296
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,143	17,975
Gross unrealized gains	$ 6,786	$ 6,719
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	$ (112)	$ (40)
Estimated market value	$ 24,817	$ 24,654