Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Mar. 31, 2023
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	$ 2,660,093	$ 3,006,587
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	6,087	5,319
Gross unrealized losses more than 12 months	(222,624)	(300,768)
Allowance for expected credit loss	(1,052)	(2,101)
Available for sale investments, estimated market value	2,442,504	2,709,037
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	191,070	353,189
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	2,123	3,061
Gross unrealized losses more than 12 months	(8,921)	(11,574)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	184,272	344,676
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	48,067	34,126
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	250	40
Gross unrealized losses more than 12 months	(7,664)	(6,935)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	40,653	27,231
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	151,197	161,960
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	918	649
Gross unrealized losses more than 12 months	(7,533)	(12,104)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	144,582	150,505
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	1,963,249	2,086,432
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	2,762	1,491
Gross unrealized losses more than 12 months	(152,799)	(216,589)
Allowance for expected credit loss	(1,052)	(2,101)
Available for sale investments, estimated market value	1,812,160	1,869,233
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Debt Securities, Available-for-Sale, Amortized Cost, Current	306,510	370,880
Available For Sale Debt Securities Accumulated Gross Unrealized Gain	34	78
Gross unrealized losses more than 12 months	(45,707)	(53,566)
Allowance for expected credit loss	0	0
Available for sale investments, estimated market value	$ 260,837	$ 317,392