Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,263,299	$ 2,334,215
Gross Unrealized Gains	206,379	135,767
Gross unrealized losses more than 12 months	(993)	(1,077)
Gross unrealized losses less than 12 months	(5,268)	(1,292)
Allowance for Expected Credit Losses	(4,258)
Estimated market value	2,459,159	2,467,613
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	92,389	112,421
Gross Unrealized Gains	13,609	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	105,998	120,379
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	110,514	88,449
Gross Unrealized Gains	2,808	759
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	0	(373)
Allowance for Expected Credit Losses	0
Estimated market value	113,321	88,834
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	257,153	287,643
Gross Unrealized Gains	26,803	20,664
Gross unrealized losses more than 12 months	(79)	(155)
Gross unrealized losses less than 12 months	0	0
Allowance for Expected Credit Losses	0
Estimated market value	283,877	308,152
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,602,921	1,656,425
Gross Unrealized Gains	151,789	100,302
Gross unrealized losses more than 12 months	(912)	(919)
Gross unrealized losses less than 12 months	(5,140)	(812)
Allowance for Expected Credit Losses	(4,258)
Estimated market value	1,744,400	1,754,996
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	198,829	187,784
Gross Unrealized Gains	11,333	6,011
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(128)	(107)
Allowance for Expected Credit Losses	0
Estimated market value	210,033	193,687
Redeemable preferred stocks [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,493	1,493
Gross Unrealized Gains	37	72
Gross unrealized losses more than 12 months	0	0
Gross unrealized losses less than 12 months	0	0
Allowance for Expected Credit Losses	0
Estimated market value	$ 1,530	$ 1,565