Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Mar. 31, 2021
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,559,271	$ 2,405,612
Gross Unrealized Gains	193,396	250,254
Gross unrealized losses more than 12 months	(107)	(224)
Gross unrealized losses less than 12 months	(9,409)	(784)
Allowance for Expected Credit Losses	0	(1,319)
Estimated market value	2,743,151	2,653,539
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	118,140	92,429
Gross Unrealized Gains	9,039	12,941
Gross unrealized losses more than 12 months	0	0
Gross unrealized losses less than 12 months	(678)	0
Allowance for Expected Credit Losses	0	0
Estimated market value	126,501	105,370
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	57,083	61,427
Gross Unrealized Gains	370	911
Gross unrealized losses more than 12 months	0	(1)
Gross unrealized losses less than 12 months	(2,439)	(132)
Allowance for Expected Credit Losses	0	0
Estimated market value	55,014	62,205
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	191,141	230,521
Gross Unrealized Gains	16,734	25,249
Gross unrealized losses more than 12 months	(80)	(59)
Gross unrealized losses less than 12 months	(474)	(3)
Allowance for Expected Credit Losses	0	0
Estimated market value	207,321	255,708
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	2,029,477	1,846,507
Gross Unrealized Gains	157,250	199,447
Gross unrealized losses more than 12 months	(26)	(163)
Gross unrealized losses less than 12 months	(5,818)	(641)
Allowance for Expected Credit Losses	0	(1,319)
Estimated market value	2,180,883	2,043,831
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	163,430	174,728
Gross Unrealized Gains	10,003	11,706
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	0	(8)
Allowance for Expected Credit Losses	0	0
Estimated market value	$ 173,432	$ 186,425