Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Mar. 31, 2022
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,989,120	$ 2,664,037
Gross unrealized gain	1,963	170,294
Gross unrealized losses more than 12 months	(53,121)	(445)
Gross unrealized losses less than 12 months	(272,191)	(12,734)
Allowance for expected credit loss	(2,243)	(60)
Available for sale investments, estimated market value	2,663,528	2,821,092
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	353,331	128,078
Gross unrealized gain	582	7,984
Gross unrealized losses more than 12 months	(3,844)	0
Gross unrealized losses less than 12 months	(7,761)	(969)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	342,308	135,093
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	34,920	44,678
Gross unrealized gain	58	280
Gross unrealized losses more than 12 months	(6,263)	(42)
Gross unrealized losses less than 12 months	(602)	(3,111)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	28,113	41,805
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	164,547	178,040
Gross unrealized gain	503	15,450
Gross unrealized losses more than 12 months	(3,458)	0
Gross unrealized losses less than 12 months	(10,609)	(508)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	150,983	192,982
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	2,060,503	1,989,212
Gross unrealized gain	790	138,909
Gross unrealized losses more than 12 months	(39,556)	(402)
Gross unrealized losses less than 12 months	(210,257)	(6,604)
Allowance for expected credit loss	(2,243)	(60)
Available for sale investments, estimated market value	1,809,237	2,121,055
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	375,819	324,029
Gross unrealized gain	30	7,671
Gross unrealized losses more than 12 months	0	(1)
Gross unrealized losses less than 12 months	(42,962)	(1,542)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	$ 332,887	$ 330,157