Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	$ 2,631,881	$ 2,660,093
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	2,052	6,087
Gross unrealized losses more than 12 months	(235,048)	(222,624)
Allowance for expected credit loss	(2,995)	(1,052)
Available for sale investments, estimated market value	2,395,890	2,442,504
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	147,881	191,070
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	235	2,123
Gross unrealized losses more than 12 months	(9,361)	(8,921)
Available for sale investments, estimated market value	138,755	184,272
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	47,208	48,067
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	153	250
Gross unrealized losses more than 12 months	(8,044)	(7,664)
Available for sale investments, estimated market value	39,317	40,653
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	150,396	151,197
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	313	918
Gross unrealized losses more than 12 months	(8,108)	(7,533)
Available for sale investments, estimated market value	142,601	144,582
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	1,987,105	1,963,249
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	1,318	2,762
Gross unrealized losses more than 12 months	(165,015)	(152,799)
Allowance for expected credit loss	(2,995)	(1,052)
Available for sale investments, estimated market value	1,820,413	1,812,160
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	299,291	306,510
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	33	34
Gross unrealized losses more than 12 months	(44,520)	(45,707)
Available for sale investments, estimated market value	$ 254,804	$ 260,837