Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,558,279	$ 2,405,612
Gross Unrealized Gains	168,014	250,254
Gross unrealized losses more than 12 months	(1,193)	(224)
Gross unrealized losses less than 12 months	(21,007)	(784)
Allowance for Expected Credit Losses	(1,041)	(1,319)
Estimated market value	2,703,052	2,653,539
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	104,016	92,429
Gross Unrealized Gains	9,087	12,941
Gross unrealized losses more than 12 months	0	0
Gross unrealized losses less than 12 months	(818)	0
Allowance for Expected Credit Losses	0	0
Estimated market value	112,285	105,370
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	70,219	61,427
Gross Unrealized Gains	782	911
Gross unrealized losses more than 12 months	(7)	(1)
Gross unrealized losses less than 12 months	(762)	(132)
Allowance for Expected Credit Losses	0	0
Estimated market value	70,232	62,205
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	215,298	230,521
Gross Unrealized Gains	18,323	25,249
Gross unrealized losses more than 12 months	(106)	(59)
Gross unrealized losses less than 12 months	(969)	(3)
Allowance for Expected Credit Losses	0	0
Estimated market value	232,546	255,708
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,998,177	1,846,507
Gross Unrealized Gains	129,442	199,447
Gross unrealized losses more than 12 months	(1,079)	(163)
Gross unrealized losses less than 12 months	(18,456)	(641)
Allowance for Expected Credit Losses	(1,041)	(1,319)
Estimated market value	2,107,043	2,043,831
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	170,569	174,728
Gross Unrealized Gains	10,380	11,706
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(2)	(8)
Allowance for Expected Credit Losses	0	0
Estimated market value	$ 180,946	$ 186,425