Investments (Available For Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Mar. 31, 2020
Available-for-sale Securities [Abstract]
Amortized cost	$ 2,351,246	$ 2,334,215
Gross Unrealized Gains	224,307	135,767
Gross unrealized losses more than 12 months	(678)	(1,077)
Gross unrealized losses less than 12 months	(5,747)	(1,292)
Allowance for Expected Credit Losses	0
Estimated market value	2,566,813	2,467,613
U.S. treasury securities and government obligations [Member]
Available-for-sale Securities [Abstract]
Amortized cost	92,300	112,421
Gross Unrealized Gains	13,787	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	106,087	120,379
U.S. government agency mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	88,190	88,449
Gross Unrealized Gains	1,208	759
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(53)	(373)
Allowance for Expected Credit Losses	0
Estimated market value	89,344	88,834
Obligations of states and political subdivisions [Member]
Available-for-sale Securities [Abstract]
Amortized cost	241,299	287,643
Gross Unrealized Gains	25,533	20,664
Gross unrealized losses more than 12 months	(63)	(155)
Gross unrealized losses less than 12 months	(26)	0
Allowance for Expected Credit Losses	0
Estimated market value	266,743	308,152
Corporate securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,725,211	1,656,425
Gross Unrealized Gains	169,856	100,302
Gross unrealized losses more than 12 months	(613)	(919)
Gross unrealized losses less than 12 months	(5,459)	(812)
Allowance for Expected Credit Losses	(2,315)
Estimated market value	1,886,680	1,754,996
Mortgage-backed securities [Member]
Available-for-sale Securities [Abstract]
Amortized cost	202,753	187,784
Gross Unrealized Gains	13,896	6,011
Gross unrealized losses more than 12 months	(1)	(1)
Gross unrealized losses less than 12 months	(209)	(107)
Allowance for Expected Credit Losses	0
Estimated market value	216,439	193,687
Redeemable preferred stocks [Member]
Available-for-sale Securities [Abstract]
Amortized cost	1,493	1,493
Gross Unrealized Gains	27	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	$ 1,520	$ 1,565