Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018
Available-for-sale securities, investments:
Amortized cost	$ 2,125,089	$ 1,847,422
Gross unrealized gains	22,345	79,262
Gross unrealized losses more than 12 months	(10,859)	(4,302)
Gross unrealized losses less than 12 months	(34,836)	(2,522)
Estimated market value	2,101,739	1,919,860
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	128,934	123,557
Gross unrealized gains	1,140	3,595
Gross unrealized losses more than 12 months	(2,319)	(1,036)
Gross unrealized losses less than 12 months	(2,462)	(203)
Estimated market value	125,293	125,913
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	61,243	36,416
Gross unrealized gains	526	951
Gross unrealized losses more than 12 months	(44)	(1)
Gross unrealized losses less than 12 months	(1,558)	(93)
Estimated market value	60,167	37,273
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	238,243	178,702
Gross unrealized gains	5,203	9,938
Gross unrealized losses more than 12 months	(251)	(217)
Gross unrealized losses less than 12 months	(2,456)	(18)
Estimated market value	240,739	188,405
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,576,170	1,388,300
Gross unrealized gains	14,898	50,056
Gross unrealized losses more than 12 months	(8,033)	(3,009)
Gross unrealized losses less than 12 months	(26,956)	(1,826)
Estimated market value	1,556,079	1,433,521
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	119,006	94,106
Gross unrealized gains	541	2,072
Gross unrealized losses more than 12 months	(212)
Gross unrealized losses less than 12 months	(1,384)	(153)
Estimated market value	117,951	96,025
Redeemable preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost	1,493	2,118
Gross unrealized gains	37
Gross unrealized losses more than 12 months
Gross unrealized losses less than 12 months	(20)
Estimated market value	$ 1,510
Preferred stocks [Member]
Available-for-sale securities, investments:
Amortized cost		10,609
Gross unrealized gains		321
Gross unrealized losses more than 12 months		(29)
Gross unrealized losses less than 12 months		(40)
Estimated market value		10,861
Common stocks [Member]
Available-for-sale securities, investments:
Amortized cost		15,732
Gross unrealized gains		12,329
Gross unrealized losses more than 12 months		(10)
Gross unrealized losses less than 12 months		(189)
Estimated market value		$ 27,862