Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016
Available-for-sale securities, investments:
Amortized cost	$ 1,603,186	$ 1,496,256
Gross unrealized gains	77,484	51,761
Gross unrealized losses more than 12 months	(7,937)	(8,110)
Gross unrealized losses less than 12 months	(16,574)	(29,369)
Estimated market value	1,656,159	1,510,538
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	98,134	85,861
Gross unrealized gains	6,159	3,791
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(50)	(193)
Estimated market value	104,243	89,459
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	30,884	21,845
Gross unrealized gains	1,741	1,596
Gross unrealized losses more than 12 months	(5)	(6)
Gross unrealized losses less than 12 months		(39)
Estimated market value	32,620	23,396
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	165,322	166,725
Gross unrealized gains	14,339	10,660
Gross unrealized losses more than 12 months	(32)	(81)
Gross unrealized losses less than 12 months	(16)	(414)
Estimated market value	179,613	176,890
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,232,287	1,143,125
Gross unrealized gains	45,160	26,861
Gross unrealized losses more than 12 months	(7,890)	(8,013)
Gross unrealized losses less than 12 months	(16,275)	(28,181)
Estimated market value	1,253,282	1,133,792
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	42,850	42,991
Gross unrealized gains	855	475
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(7)	(62)
Estimated market value	43,698	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,977	17,977
Gross unrealized gains	530	556
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(191)	(105)
Estimated market value	16,316	18,428
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,732	17,732
Gross unrealized gains	8,700	7,822
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months	(35)	(375)
Estimated market value	$ 26,387	$ 25,169