Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,317,006	$ 2,334,215
Gross Unrealized Gains	105,872	135,767
Gross unrealized losses more than 12 months	(1,903)	(1,077)
Gross unrealized losses less than 12 months	(43,245)	(1,292)
Allowance for Expected Credit Losses	(5,407)
Estimated market value	2,372,323	2,467,613
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	84,814	112,421
Gross Unrealized Gains	12,192	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	97,006	120,379
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	126,599	88,449
Gross Unrealized Gains	3,458	759
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(39)	(373)
Allowance for Expected Credit Losses	0
Estimated market value	130,017	88,834
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	270,476	287,643
Gross Unrealized Gains	21,364	20,664
Gross unrealized losses more than 12 months	(141)	(155)
Gross unrealized losses less than 12 months	(2)	0
Allowance for Expected Credit Losses	0
Estimated market value	291,697	308,152
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,636,025	1,656,425
Gross Unrealized Gains	66,359	100,302
Gross unrealized losses more than 12 months	(1,759)	(919)
Gross unrealized losses less than 12 months	(37,168)	(812)
Allowance for Expected Credit Losses	(5,407)
Estimated market value	1,658,050	1,754,996
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	197,599	187,784
Gross Unrealized Gains	2,485	6,011
Gross unrealized losses more than 12 months	(2)	(1)
Gross unrealized losses less than 12 months	(6,031)	(107)
Allowance for Expected Credit Losses	0
Estimated market value	194,051	193,687
Redeemable preferred stocks [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,493	1,493
Gross Unrealized Gains	14	72
Gross unrealized losses more than 12 months	0	0
Gross unrealized losses less than 12 months	(5)	0
Allowance for Expected Credit Losses	0
Estimated market value	$ 1,502	$ 1,565