Investments (Summary Of Unrealized Appreciation (Depreciation) Of Available For Sale, Fixed Maturity And Equity Security Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Allowance for credit losses	$ (23,783)	$ (8,708)	$ (1,566)	$ (19,925)	$ (12,099)
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,255,282		10,248,650
Allowance for credit losses	(23,783)		(1,566)
Unrealized Appreciation	364,892		442,433
Unrealized Depreciation	(42,574)		(45,952)
Market Value	11,553,817		10,643,565
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	538,733		659,957
Allowance for credit losses
Unrealized Appreciation	14,207		22,032
Unrealized Depreciation	(106)
Market Value	552,834		681,989
Obligation of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	573,398		543,646
Allowance for credit losses
Unrealized Appreciation	35,932		34,655
Unrealized Depreciation	(986)		(1,255)
Market Value	608,344		577,046
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,539,895		3,316,525
Allowance for credit losses	(18,475)	(3,588)	(1,205)	(19,398)	(11,468)
Unrealized Appreciation	131,825		166,072
Unrealized Depreciation	(23,239)		(31,480)
Market Value	3,630,006		3,449,912
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,989,971		2,450,807
Allowance for credit losses	(4,915)	(4,915)
Unrealized Appreciation	32,904		28,585
Unrealized Depreciation	(2,426)		(5,222)
Market Value	3,015,534		2,474,170
Commercial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	575,995		512,388
Allowance for credit losses
Unrealized Appreciation	28,512		37,875
Unrealized Depreciation	(2,478)		(183)
Market Value	602,029		550,080
Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,019,028		937,166
Allowance for credit losses
Unrealized Appreciation	21,091		28,630
Unrealized Depreciation	(6,168)		(696)
Market Value	1,033,951		965,100
Non-agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,633		3,164
Allowance for credit losses
Unrealized Appreciation	4		2
Unrealized Depreciation	(2)		(2)
Market Value	5,635		3,164
Foreign Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	717,562		694,132
Allowance for credit losses					(70)
Unrealized Appreciation	41,661		51,317
Unrealized Depreciation	(2,571)		(3,211)
Market Value	756,652		742,238
Foreign Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,295,067		1,130,865
Allowance for credit losses	(393)	$ (205)	(361)	$ (527)	$ (561)
Unrealized Appreciation	58,756		73,265
Unrealized Depreciation	(4,600)		(3,903)
Market Value	$ 1,348,830		$ 1,199,866