Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
Available-for-sale securities, investments:
Amortized cost	$ 1,601,188	$ 1,496,256
Gross unrealized gains	105,561	51,761
Gross unrealized losses more than 12 months	(4,431)	(8,110)
Gross unrealized losses less than 12 months	(1,737)	(29,369)
Estimated market value	1,700,581	1,510,538
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	87,116	85,861
Gross unrealized gains	6,872	3,791
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(80)	(193)
Estimated market value	93,908	89,459
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	28,935	21,845
Gross unrealized gains	1,940	1,596
Gross unrealized losses more than 12 months	(5)	(6)
Gross unrealized losses less than 12 months	(2)	(39)
Estimated market value	30,868	23,396
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	159,596	166,725
Gross unrealized gains	15,829	10,660
Gross unrealized losses more than 12 months	(12)	(81)
Gross unrealized losses less than 12 months	(90)	(414)
Estimated market value	175,323	176,890
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,250,266	1,143,125
Gross unrealized gains	70,953	26,861
Gross unrealized losses more than 12 months	(4,404)	(8,013)
Gross unrealized losses less than 12 months	(1,411)	(28,181)
Estimated market value	1,315,404	1,133,792
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	42,665	42,991
Gross unrealized gains	1,214	475
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(58)	(62)
Estimated market value	43,821	43,404
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	14,640	17,977
Gross unrealized gains	492	556
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(96)	(105)
Estimated market value	15,036	18,428
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,970	17,732
Gross unrealized gains	8,261	7,822
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months		(375)
Estimated market value	$ 26,221	$ 25,169