Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Available-for-sale securities, investments:
Amortized cost	$ 1,496,256	$ 1,237,386
Gross unrealized gains	51,761	73,796
Gross unrealized losses more than 12 months	(8,110)	(3,177)
Gross unrealized losses less than 12 months	(29,369)	(3,043)
Estimated market value	1,510,538	1,304,962
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	85,861	99,722
Gross unrealized gains	$ 3,791	5,658
Gross unrealized losses more than 12 months		$ (64)
Gross unrealized losses less than 12 months	$ (193)
Estimated market value	89,459	$ 105,316
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	21,845	30,569
Gross unrealized gains	1,596	2,614
Gross unrealized losses more than 12 months	(6)	(39)
Gross unrealized losses less than 12 months	(39)	(3)
Estimated market value	23,396	33,141
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	166,725	165,724
Gross unrealized gains	10,660	13,052
Gross unrealized losses more than 12 months	(81)	(298)
Gross unrealized losses less than 12 months	(414)	(10)
Estimated market value	176,890	178,468
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,143,125	885,470
Gross unrealized gains	26,861	44,426
Gross unrealized losses more than 12 months	(8,013)	(2,522)
Gross unrealized losses less than 12 months	(28,181)	(2,966)
Estimated market value	1,133,792	924,408
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	42,991	19,874
Gross unrealized gains	$ 475	806
Gross unrealized losses more than 12 months		$ (1)
Gross unrealized losses less than 12 months	$ (62)
Estimated market value	43,404	$ 20,679
Redeemable Preferred Stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,977	18,052
Gross unrealized gains	$ 556	521
Gross unrealized losses more than 12 months		(253)
Gross unrealized losses less than 12 months	$ (105)	(24)
Estimated market value	18,428	18,296
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	17,732	17,975
Gross unrealized gains	7,822	$ 6,719
Gross unrealized losses more than 12 months	(10)
Gross unrealized losses less than 12 months	(375)	$ (40)
Estimated market value	$ 25,169	$ 24,654