Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Mar. 31, 2023
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,758,446	$ 3,006,587
Gross unrealized gain	1,966	5,319
Gross unrealized losses more than 12 months	(204,850)	(118,943)
Gross unrealized losses less than 12 months	(81,737)	(181,825)
Allowance for expected credit loss	(1,697)	(2,101)
Estimated market value	2,472,128	2,709,037
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	172,987	353,189
Gross unrealized gain	335	3,061
Gross unrealized losses more than 12 months	(2,315)	(7,639)
Gross unrealized losses less than 12 months	(116)	(3,935)
Allowance for expected credit loss	0	0
Estimated market value	170,891	344,676
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	99,230	34,126
Gross unrealized gain	1	40
Gross unrealized losses more than 12 months	(3,703)	(6,707)
Gross unrealized losses less than 12 months	(5,009)	(228)
Allowance for expected credit loss	0	0
Estimated market value	90,519	27,231
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	154,547	161,960
Gross unrealized gain	441	649
Gross unrealized losses more than 12 months	(4,261)	(4,014)
Gross unrealized losses less than 12 months	(5,064)	(8,090)
Allowance for expected credit loss	0	0
Estimated market value	145,663	150,505
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,817,003	2,086,432
Gross unrealized gain	983	1,491
Gross unrealized losses more than 12 months	(154,177)	(60,224)
Gross unrealized losses less than 12 months	(47,305)	(156,365)
Allowance for expected credit loss	(1,697)	(2,101)
Estimated market value	1,614,807	1,869,233
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	514,679	370,880
Gross unrealized gain	206	78
Gross unrealized losses more than 12 months	(40,394)	(40,359)
Gross unrealized losses less than 12 months	(24,243)	(13,207)
Allowance for expected credit loss	0	0
Estimated market value	$ 450,248	$ 317,392