Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Mar. 31, 2016
Available-for-sale securities, investments:
Amortized cost	$ 1,539,357	$ 1,496,256
Gross unrealized gains	104,315	51,761
Gross unrealized losses more than 12 months	(7,802)	(8,110)
Gross unrealized losses less than 12 months	(3,450)	(29,369)
Estimated market value	1,632,420	1,510,538
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	73,751	85,861
Gross unrealized gains	7,216	3,791
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months		(193)
Estimated market value	80,967	89,459
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	29,902	21,845
Gross unrealized gains	2,116	1,596
Gross unrealized losses more than 12 months	(6)	(6)
Gross unrealized losses less than 12 months	(2)	(39)
Estimated market value	32,010	23,396
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	161,620	166,725
Gross unrealized gains	16,285	10,660
Gross unrealized losses more than 12 months	(11)	(81)
Gross unrealized losses less than 12 months		(414)
Estimated market value	177,894	176,890
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,198,017	1,143,125
Gross unrealized gains	67,183	26,861
Gross unrealized losses more than 12 months	(7,775)	(8,013)
Gross unrealized losses less than 12 months	(3,266)	(28,181)
Estimated market value	1,254,159	1,133,792
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	42,765	42,991
Gross unrealized gains	1,238	475
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(38)	(62)
Estimated market value	43,965	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,332	17,977
Gross unrealized gains	795	556
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(127)	(105)
Estimated market value	16,000	18,428
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,970	17,732
Gross unrealized gains	9,482	7,822
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months	(17)	(375)
Estimated market value	$ 27,425	$ 25,169