Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Mar. 31, 2015
Available-for-sale securities, investments:
Amortized cost	$ 1,420,928	$ 1,237,386
Gross unrealized gains	62,589	73,796
Gross unrealized losses more than 12 months	(2,332)	(3,177)
Gross unrealized losses less than 12 months	(17,188)	(3,043)
Estimated market value	1,463,997	1,304,962
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	76,429	99,722
Gross unrealized gains	$ 5,155	5,658
Gross unrealized losses more than 12 months		$ (64)
Gross unrealized losses less than 12 months
Estimated market value	$ 81,584	$ 105,316
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	22,875	30,569
Gross unrealized gains	1,959	2,614
Gross unrealized losses more than 12 months	$ (5)	(39)
Gross unrealized losses less than 12 months		(3)
Estimated market value	$ 24,829	33,141
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	168,889	165,724
Gross unrealized gains	12,156	13,052
Gross unrealized losses more than 12 months	(58)	(298)
Gross unrealized losses less than 12 months	(264)	(10)
Estimated market value	180,723	178,468
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,073,111	885,470
Gross unrealized gains	36,145	44,426
Gross unrealized losses more than 12 months	(2,259)	(2,522)
Gross unrealized losses less than 12 months	(16,496)	(2,966)
Estimated market value	1,090,501	924,408
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	43,076	19,874
Gross unrealized gains	$ 1,014	806
Gross unrealized losses more than 12 months		$ (1)
Gross unrealized losses less than 12 months
Estimated market value	$ 44,090	$ 20,679
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,056	18,052
Gross unrealized gains	$ 312	521
Gross unrealized losses more than 12 months		(253)
Gross unrealized losses less than 12 months	$ (198)	(24)
Estimated market value	18,170	18,296
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	18,492	17,975
Gross unrealized gains	5,848	$ 6,719
Gross unrealized losses more than 12 months	(10)
Gross unrealized losses less than 12 months	(230)	$ (40)
Estimated market value	$ 24,100	$ 24,654