Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Mar. 31, 2024
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	$ 2,776,940	$ 2,660,093
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	9,980	6,087
Gross unrealized losses more than 12 months	(166,745)	(222,624)
Allowance for expected credit loss	(3,049)	(1,052)
Available for sale investments, estimated market value	2,617,126	2,442,504
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	141,224	191,070
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	371	2,123
Gross unrealized losses more than 12 months	(6,094)	(8,921)
Available for sale investments, estimated market value	135,501	184,272
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	86,443	48,067
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	229	250
Gross unrealized losses more than 12 months	(7,944)	(7,664)
Available for sale investments, estimated market value	78,728	40,653
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	140,373	151,197
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	758	918
Gross unrealized losses more than 12 months	(5,468)	(7,533)
Available for sale investments, estimated market value	135,663	144,582
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	1,879,165	1,963,249
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	5,643	2,762
Gross unrealized losses more than 12 months	(111,690)	(152,799)
Allowance for expected credit loss	(3,049)	(1,052)
Available for sale investments, estimated market value	1,770,069	1,812,160
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	529,735	306,510
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	2,979	34
Gross unrealized losses more than 12 months	(35,549)	(45,707)
Available for sale investments, estimated market value	$ 497,165	$ 260,837