Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,640,198	$ 2,664,037
Gross Unrealized Gains	49,434	170,294
Gross unrealized losses more than 12 months	(14,493)	(445)
Gross unrealized losses less than 12 months	(76,863)	(12,734)
Allowance for Expected Credit Losses	(77)	(60)
Estimated market value	2,598,199	2,821,092
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	127,940	128,078
Gross Unrealized Gains	2,947	7,984
Gross unrealized losses more than 12 months	(1,578)	0
Gross unrealized losses less than 12 months	(2,493)	(969)
Allowance for Expected Credit Losses	0	0
Estimated market value	126,816	135,093
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	37,806	44,678
Gross Unrealized Gains	149	280
Gross unrealized losses more than 12 months	(380)	(42)
Gross unrealized losses less than 12 months	(4,126)	(3,111)
Allowance for Expected Credit Losses	0	0
Estimated market value	33,449	41,805
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	166,572	178,040
Gross Unrealized Gains	5,830	15,450
Gross unrealized losses more than 12 months	(1,275)	0
Gross unrealized losses less than 12 months	(2,009)	(508)
Allowance for Expected Credit Losses	0	0
Estimated market value	169,118	192,982
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,970,857	1,989,212
Gross Unrealized Gains	38,302	138,909
Gross unrealized losses more than 12 months	(11,259)	(402)
Gross unrealized losses less than 12 months	(51,863)	(6,604)
Allowance for Expected Credit Losses	(77)	(60)
Estimated market value	1,945,960	2,121,055
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	337,023	324,029
Gross Unrealized Gains	2,206	7,671
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(16,372)	(1,542)
Allowance for Expected Credit Losses	0	0
Estimated market value	$ 322,856	$ 330,157