Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	$ 2,668,172	$ 3,006,587
Gross Unrealized Gains	3,842	5,319
Gross unrealized losses more than 12 months	(215,560)	(118,943)
Gross unrealized losses less than 12 months	(47,582)	(181,825)
Allowance for Expected Credit Losses	(1,646)	(2,101)
Fair Value	2,407,226	2,709,037
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	128,050	353,189
Gross Unrealized Gains	391	3,061
Gross unrealized losses more than 12 months	(7,620)	(7,639)
Gross unrealized losses less than 12 months	(1,973)	(3,935)
Allowance for Expected Credit Losses	0	0
Fair Value	118,848	344,676
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	33,769	34,126
Gross Unrealized Gains	55	40
Gross unrealized losses more than 12 months	(6,810)	(6,707)
Gross unrealized losses less than 12 months	(62)	(228)
Allowance for Expected Credit Losses	0	0
Fair Value	26,952	27,231
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	159,833	161,960
Gross Unrealized Gains	1,060	649
Gross unrealized losses more than 12 months	(6,024)	(4,014)
Gross unrealized losses less than 12 months	(2,788)	(8,090)
Allowance for Expected Credit Losses	0	0
Fair Value	152,081	150,505
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	1,987,107	2,086,432
Gross Unrealized Gains	2,253	1,491
Gross unrealized losses more than 12 months	(147,453)	(60,224)
Gross unrealized losses less than 12 months	(39,283)	(156,365)
Allowance for Expected Credit Losses	(1,646)	(2,101)
Fair Value	1,800,978	1,869,233
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost, equity investments	359,413	370,880
Gross Unrealized Gains	83	78
Gross unrealized losses more than 12 months	(47,653)	(40,359)
Gross unrealized losses less than 12 months	(3,476)	(13,207)
Allowance for Expected Credit Losses	0	0
Fair Value	$ 308,367	$ 317,392