Investments (Summary Of Unrealized Appreciation (Depreciation) Of Available For Sale, Fixed Maturity And Equity Security Investments) (Details) - USD ($)	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Allowance for credit losses	$ (19,925,000)	$ (12,099,000)	$ 0
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,657,782,000		7,334,425,000
Allowance for credit losses	(19,925,000)
Unrealized Appreciation	371,929,000		203,138,000
Unrealized Depreciation	(90,409,000)		(45,484,000)
Market Value	7,919,377,000		7,492,079,000
OTTI in AOCI			605,000
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	741,244,000		768,374,000
Allowance for credit losses
Unrealized Appreciation	31,740,000		10,128,000
Unrealized Depreciation			(987,000)
Market Value	772,984,000		777,515,000
OTTI in AOCI
Obligation of U.S. States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	507,475,000		506,347,000
Allowance for credit losses
Unrealized Appreciation	27,597,000		29,651,000
Unrealized Depreciation	(3,789,000)		(87,000)
Market Value	531,283,000		535,911,000
OTTI in AOCI
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,935,665,000		2,777,097,000
Allowance for credit losses	(19,398,000)	(11,468,000)
Unrealized Appreciation	135,213,000		70,898,000
Unrealized Depreciation	(51,922,000)		(26,438,000)
Market Value	2,999,558,000		2,821,557,000
OTTI in AOCI			245,000
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	991,779,000		761,607,000
Allowance for credit losses
Unrealized Appreciation	14,504,000		5,659,000
Unrealized Depreciation	(21,500,000)		(1,309,000)
Market Value	984,783,000		765,957,000
OTTI in AOCI
Commercial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	348,329,000		311,961,000
Allowance for credit losses
Unrealized Appreciation	33,506,000		17,242,000
Unrealized Depreciation	(788,000)		(154,000)
Market Value	381,047,000		329,049,000
OTTI in AOCI
Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	604,360,000		625,612,000
Allowance for credit losses
Unrealized Appreciation	33,190,000		19,395,000
Unrealized Depreciation	(349,000)		(320,000)
Market Value	637,201,000		644,687,000
OTTI in AOCI
Non-agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	509,000		1,638,000
Allowance for credit losses
Unrealized Appreciation
Unrealized Depreciation	(3,000)
Market Value	506,000		1,638,000
OTTI in AOCI
Foreign Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	636,346,000		646,149,000
Allowance for credit losses		(70,000)
Unrealized Appreciation	38,578,000		18,908,000
Unrealized Depreciation	(5,034,000)		(7,050,000)
Market Value	669,890,000		658,007,000
OTTI in AOCI			27,000
Foreign Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	892,075,000		935,640,000
Allowance for credit losses	(527,000)	$ (561,000)
Unrealized Appreciation	57,601,000		31,257,000
Unrealized Depreciation	(7,024,000)		(9,139,000)
Market Value	$ 942,125,000		957,758,000
OTTI in AOCI			$ 333,000