Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Mar. 31, 2022
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,747,833	$ 2,664,037
Gross unrealized gain	8,619	170,294
Gross unrealized losses more than 12 months	(25,247)	(445)
Gross unrealized losses less than 12 months	(176,886)	(12,734)
Allowance for expected credit loss	(2,008)	(60)
Estimated market value	2,552,311	2,821,092
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	127,788	128,078
Gross unrealized gain	735	7,984
Gross unrealized losses more than 12 months	(2,289)	0
Gross unrealized losses less than 12 months	(4,121)	(969)
Allowance for expected credit loss	0	0
Estimated market value	122,113	135,093
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	36,023	44,678
Gross unrealized gain	102	280
Gross unrealized losses more than 12 months	(3,440)	(42)
Gross unrealized losses less than 12 months	(1,897)	(3,111)
Allowance for expected credit loss	0	0
Estimated market value	30,788	41,805
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	165,048	178,040
Gross unrealized gain	1,853	15,450
Gross unrealized losses more than 12 months	(1,759)	0
Gross unrealized losses less than 12 months	(5,521)	(508)
Allowance for expected credit loss	0	0
Estimated market value	159,621	192,982
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	2,054,320	1,989,212
Gross unrealized gain	5,535	138,909
Gross unrealized losses more than 12 months	(17,759)	(402)
Gross unrealized losses less than 12 months	(134,689)	(6,604)
Allowance for expected credit loss	(2,008)	(60)
Estimated market value	1,905,399	2,121,055
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	364,654	324,029
Gross unrealized gain	394	7,671
Gross unrealized losses more than 12 months	0	(1)
Gross unrealized losses less than 12 months	(30,658)	(1,542)
Allowance for expected credit loss	0	0
Estimated market value	$ 334,390	$ 330,157