Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017
Available-for-sale securities, investments:
Amortized cost	$ 1,847,422	$ 1,630,600
Gross unrealized gains	79,262	55,740
Gross unrealized losses more than 12 months	(4,302)	(5,770)
Gross unrealized losses less than 12 months	(2,522)	(16,802)
Estimated market value	1,919,860	1,663,768
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	123,557	123,474
Gross unrealized gains	3,595	2,892
Gross unrealized losses more than 12 months	(1,036)
Gross unrealized losses less than 12 months	(203)	(1,675)
Estimated market value	125,913	124,691
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	36,416	27,908
Gross unrealized gains	951	1,070
Gross unrealized losses more than 12 months	(1)	(6)
Gross unrealized losses less than 12 months	(93)	(377)
Estimated market value	37,273	28,595
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	178,702	159,417
Gross unrealized gains	9,938	9,466
Gross unrealized losses more than 12 months	(217)	(23)
Gross unrealized losses less than 12 months	(18)	(424)
Estimated market value	188,405	168,436
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,388,300	1,263,703
Gross unrealized gains	50,056	32,901
Gross unrealized losses more than 12 months	(3,009)	(5,731)
Gross unrealized losses less than 12 months	(1,826)	(13,837)
Estimated market value	1,433,521	1,277,036
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	94,106	26,577
Gross unrealized gains	2,072	515
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(153)	(5)
Estimated market value	96,025	27,087
Redeemable Preferred Stocks [Member]
Available-for-sale securities, investments:
Amortized cost	10,609	13,789
Gross unrealized gains	321	168
Gross unrealized losses more than 12 months	(29)
Gross unrealized losses less than 12 months	(40)	(468)
Estimated market value	10,861	13,489
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost	15,732	15,732
Gross unrealized gains	12,329	8,728
Gross unrealized losses more than 12 months	(10)	(10)
Gross unrealized losses less than 12 months	(189)	(16)
Estimated market value	$ 27,862	$ 24,434