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]
Amortized Cost	$ 16,392,238,000
Allowance for Credit Losses	(25,837,000)	$ (21,774,000)	$ 0
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,392,238,000		16,473,491,000
Allowance for Credit Losses	(25,837,000)
Unrealized Appreciation	858,245,000		472,622,000
Unrealized Depreciation	(169,387,000)		(121,169,000)
Market Value	17,055,259,000		16,824,944,000
OTTI in AOCI			987,000
U.S. Treasury Securities And Obligations Of U.S. Government Agencies And Corporations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,355,970,000		1,489,660,000
Allowance for Credit Losses
Unrealized Appreciation	79,243,000		28,357,000
Unrealized Depreciation	(282,000)		(2,214,000)
Market Value	1,434,931,000		1,515,803,000
OTTI in AOCI
Obligations Of U.S. States And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	508,478,000		507,353,000
Allowance for Credit Losses
Unrealized Appreciation	27,597,000		29,651,000
Unrealized Depreciation	(3,794,000)		(89,000)
Market Value	532,281,000		536,915,000
OTTI in AOCI
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,349,631,000		6,227,661,000
Allowance for Credit Losses	(22,253,000)	(17,305,000)
Unrealized Appreciation	356,702,000		185,052,000
Unrealized Depreciation	(66,292,000)		(37,767,000)
Market Value	6,617,788,000		6,374,946,000
OTTI in AOCI			469,000
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,076,648,000		892,373,000
Allowance for Credit Losses
Unrealized Appreciation	16,728,000		6,818,000
Unrealized Depreciation	(23,818,000)		(1,858,000)
Market Value	1,069,558,000		897,333,000
OTTI in AOCI
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	850,616,000		814,570,000
Allowance for Credit Losses
Unrealized Appreciation	71,017,000		31,236,000
Unrealized Depreciation	(1,837,000)		(1,249,000)
Market Value	919,796,000		844,557,000
OTTI in AOCI
Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,016,481,000		2,173,099,000
Allowance for Credit Losses
Unrealized Appreciation	74,813,000		36,361,000
Unrealized Depreciation	(2,043,000)		(10,879,000)
Market Value	2,089,251,000		2,198,581,000
OTTI in AOCI
Non-Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,153,000		5,723,000
Allowance for Credit Losses
Unrealized Appreciation
Unrealized Depreciation	(42,000)		(20,000)
Market Value	3,111,000		5,703,000
OTTI in AOCI
Foreign Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,461,380,000		1,492,315,000
Allowance for Credit Losses	(92,000)	(519,000)
Unrealized Appreciation	73,849,000		47,148,000
Unrealized Depreciation	(33,937,000)		(33,513,000)
Market Value	1,501,200,000		1,505,950,000
OTTI in AOCI			71,000
Foreign Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,769,881,000		2,870,737,000
Allowance for Credit Losses	(3,492,000)	$ (3,950,000)
Unrealized Appreciation	158,296,000		107,999,000
Unrealized Depreciation	(37,342,000)		(33,580,000)
Market Value	$ 2,887,343,000		2,945,156,000
OTTI in AOCI			$ 447,000