Investments (Summary Of Amortized Cost And Market Value Of Fixed Maturity Securities, By Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Amortized Cost	$ 17,131,414	$ 16,473,491
Fixed maturity securities - available for sale, Market Value	17,856,377	16,824,944
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Due in one year or less, Amortized Cost	1,475,335	1,456,960
Fixed maturity securities - available for sale, Due after one year through five years, Amortized Cost	6,408,491	6,757,107
Fixed maturity securities - available for sale, Due after five years through ten years, Amortized Cost	3,878,019	3,471,370
Fixed maturity securities - available for sale, Due after ten years, Amortized Cost	1,102,257	902,289
Fixed maturity securities - available for sale, Due in one year or less, Market Value	1,483,621	1,457,919
Fixed maturity securities - available for sale, Due after one year through five years, Market Value	6,624,753	6,869,359
Fixed maturity securities - available for sale, Due after five years through ten years, Market Value	4,186,765	3,609,816
Fixed maturity securities - available for sale, Due after ten years, Market Value	1,132,543	941,676
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Amortized Cost	1,326,918	892,373
Fixed maturity securities - available for sale, Market Value	1,338,202	897,333
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Amortized Cost	892,998	814,570
Fixed maturity securities - available for sale, Market Value	969,320	844,557
Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Amortized Cost	2,044,837	2,173,099
Fixed maturity securities - available for sale, Market Value	2,118,653	2,198,581
Non-Agency Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities - available for sale, Amortized Cost	2,559	5,723
Fixed maturity securities - available for sale, Market Value	$ 2,520	$ 5,703

Amount of debt securities categorized neither as held-to-maturity nor trading, held at market value.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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