Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Available-for-sale securities, investments:
Amortized cost	$ 1,630,600	$ 1,476,507
Gross unrealized gains	55,740	51,761
Gross unrealized losses more than 12 months	(5,770)	(8,110)
Gross unrealized losses less than 12 months	(16,802)	(29,369)
Estimated market value	1,663,768	1,490,789
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	123,474	85,861
Gross unrealized gains	2,892	3,791
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(1,675)	(193)
Estimated market value	124,691	89,459
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	27,908	21,845
Gross unrealized gains	1,070	1,596
Gross unrealized losses more than 12 months	(6)	(6)
Gross unrealized losses less than 12 months	(377)	(39)
Estimated market value	28,595	23,396
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	159,417	166,725
Gross unrealized gains	9,466	10,660
Gross unrealized losses more than 12 months	(23)	(81)
Gross unrealized losses less than 12 months	(424)	(414)
Estimated market value	168,436	176,890
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,263,703	1,143,125
Gross unrealized gains	32,901	26,861
Gross unrealized losses more than 12 months	(5,731)	(8,013)
Gross unrealized losses less than 12 months	(13,837)	(28,181)
Estimated market value	1,277,036	1,133,792
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	26,577	24,991
Gross unrealized gains	515	475
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(5)	(62)
Estimated market value	27,087	25,404
Redeemable Preferred Stocks [Member]
Available-for-sale securities, investments:
Amortized cost	13,789	17,977
Gross unrealized gains	168	556
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(468)	(105)
Estimated market value	13,489	18,428
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,732	15,983
Gross unrealized gains	8,728	7,822
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months	(16)	(375)
Estimated market value	$ 24,434	$ 23,420