Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Mar. 31, 2022
Available-for-sale securities, investments:
Amortized cost, equity investments	$ 3,006,587	$ 2,664,037
Gross unrealized gains	5,319	170,294
Gross unrealized losses more than 12 months	(118,943)	(445)
Gross unrealized losses less than 12 months	(181,825)	(12,734)
Allowance for Expected Credit Losses	(2,101)	(60)
Fair Value	2,709,037	2,821,092
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost, equity investments	353,189	128,078
Gross unrealized gains	3,061	7,984
Gross unrealized losses more than 12 months	(7,639)	0
Gross unrealized losses less than 12 months	(3,935)	(969)
Allowance for Expected Credit Losses	0	0
Fair Value	344,676	135,093
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost, equity investments	34,126	44,678
Gross unrealized gains	40	280
Gross unrealized losses more than 12 months	(6,707)	(42)
Gross unrealized losses less than 12 months	(228)	(3,111)
Allowance for Expected Credit Losses	0	0
Fair Value	27,231	41,805
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost, equity investments	161,960	178,040
Gross unrealized gains	649	15,450
Gross unrealized losses more than 12 months	(4,014)	0
Gross unrealized losses less than 12 months	(8,090)	(508)
Allowance for Expected Credit Losses	0	0
Fair Value	150,505	192,982
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost, equity investments	2,086,432	1,989,212
Gross unrealized gains	1,491	138,909
Gross unrealized losses more than 12 months	(60,224)	(402)
Gross unrealized losses less than 12 months	(156,365)	(6,604)
Allowance for Expected Credit Losses	(2,101)	(60)
Fair Value	1,869,233	2,121,055
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost, equity investments	370,880	324,029
Gross unrealized gains	78	7,671
Gross unrealized losses more than 12 months	(40,359)	(1)
Gross unrealized losses less than 12 months	(13,207)	(1,542)
Allowance for Expected Credit Losses	0	0
Fair Value	$ 317,392	$ 330,157