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Investments (Summary Of Amortized Cost And Market Value Of Fixed Maturity Securities, By Contractual Maturity) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturity securities - available for sale, Amortized Cost $ 19,932,190 $ 19,225,067
Fixed maturity securities - available for sale, Market Value 20,407,480 20,040,173
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 1,548,669 1,365,793
Fixed maturity securities - available for sale, Due after one year through five years, Amortized Cost 6,510,731 6,529,189
Fixed maturity securities - available for sale, Due after five years through ten years, Amortized Cost 4,895,574 4,414,211
Fixed maturity securities - available for sale, Due after ten years, Amortized Cost 1,102,085 1,247,816
Fixed maturity securities - available for sale, Due in one year or less, Market Value 1,559,198 1,374,674
Fixed maturity securities - available for sale, Due after one year through five years, Market Value 6,715,932 6,774,785
Fixed maturity securities - available for sale, Due after five years through ten years, Market Value 5,036,321 4,751,903
Fixed maturity securities - available for sale, Due after ten years, Market Value 1,122,723 1,309,773
Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturity securities - available for sale, Amortized Cost 2,615,962 2,540,809
Fixed maturity securities - available for sale, Market Value 2,635,771 2,565,802
Commercial Mortgage-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturity securities - available for sale, Amortized Cost 992,442 915,923
Fixed maturity securities - available for sale, Market Value 1,029,317 990,303
Agency residential [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturity securities - available for sale, Amortized Cost 2,257,474 2,206,139
Fixed maturity securities - available for sale, Market Value 2,298,973 2,267,739
Non-agency residential [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Fixed maturity securities - available for sale, Amortized Cost 9,253 5,187
Fixed maturity securities - available for sale, Market Value $ 9,245 $ 5,194