Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Mar. 31, 2021
Available-for-sale securities, investments:
Amortized cost	$ 2,664,037	$ 2,405,612
Gross unrealized gains	170,294	250,254
Gross unrealized losses more than 12 months	(445)	(224)
Gross unrealized losses less than 12 months	(12,734)	(783)
Allowance for Expected Credit Losses	(60)	(1,320)
Estimated market value	2,821,092	2,653,539
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	128,078	92,429
Gross unrealized gains	7,984	12,941
Gross unrealized losses more than 12 months	0	0
Gross unrealized losses less than 12 months	(969)	0
Allowance for Expected Credit Losses	0	0
Estimated market value	135,093	105,370
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	44,678	61,427
Gross unrealized gains	280	911
Gross unrealized losses more than 12 months	(42)	(1)
Gross unrealized losses less than 12 months	(3,111)	(132)
Allowance for Expected Credit Losses	0	0
Estimated market value	41,805	62,205
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	178,040	230,521
Gross unrealized gains	15,450	25,249
Gross unrealized losses more than 12 months	0	(59)
Gross unrealized losses less than 12 months	(508)	(3)
Allowance for Expected Credit Losses	0	0
Estimated market value	192,982	255,708
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,989,212	1,846,507
Gross unrealized gains	138,909	199,447
Gross unrealized losses more than 12 months	(402)	(163)
Gross unrealized losses less than 12 months	(6,604)	(640)
Allowance for Expected Credit Losses	(60)	(1,320)
Estimated market value	2,121,055	2,043,831
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	324,029	174,728
Gross unrealized gains	7,671	11,706
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(1,542)	(8)
Allowance for Expected Credit Losses	0	0
Estimated market value	$ 330,157	$ 186,425