Investments (Available-for-sale investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020
Available-for-sale securities, investments:
Amortized cost	$ 2,405,612	$ 2,334,215
Gross unrealized gains	250,254	135,767
Gross unrealized losses more than 12 months	(224)	(1,077)
Gross unrealized losses less than 12 months	(784)	(1,292)
Allowance for Expected Credit Losses	(1,319)
Estimated market value	2,653,539	2,467,613
U.S. treasury securities and government obligations [Member]
Available-for-sale securities, investments:
Amortized cost	92,429	112,421
Gross unrealized gains	12,941	7,959
Gross unrealized losses more than 12 months	0	(1)
Gross unrealized losses less than 12 months	0	0
Allowance for Expected Credit Losses	0
Estimated market value	105,370	120,379
U.S. government agency mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	61,427	88,449
Gross unrealized gains	911	759
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(132)	(373)
Allowance for Expected Credit Losses	0
Estimated market value	62,205	88,834
Obligations of states and political subdivisions [Member]
Available-for-sale securities, investments:
Amortized cost	230,521	287,643
Gross unrealized gains	25,249	20,664
Gross unrealized losses more than 12 months	(59)	(155)
Gross unrealized losses less than 12 months	(3)	0
Allowance for Expected Credit Losses	0
Estimated market value	255,708	308,152
Corporate securities [Member]
Available-for-sale securities, investments:
Amortized cost	1,846,507	1,656,425
Gross unrealized gains	199,447	100,302
Gross unrealized losses more than 12 months	(163)	(919)
Gross unrealized losses less than 12 months	(641)	(812)
Allowance for Expected Credit Losses	(1,319)
Estimated market value	2,043,831	1,754,996
Mortgage-backed securities [Member]
Available-for-sale securities, investments:
Amortized cost	174,728	187,784
Gross unrealized gains	11,706	6,011
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(8)	(107)
Allowance for Expected Credit Losses	0
Estimated market value	186,425	193,687
Redeemable Preferred Stocks [Member]
Available-for-sale securities, investments:
Amortized cost	$ 0	1,493
Gross unrealized gains		72
Gross unrealized losses more than 12 months		0
Gross unrealized losses less than 12 months		0
Estimated market value		$ 1,565