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Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis $ 22,890,985 $ 21,966,803
Available-for-sale Securities 23,520,469 23,210,968
Canadian Province Of Quebec Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 1,106,753 939,837
Available-for-sale Securities 1,863,568 1,282,944
Canadian Province Of Ontario Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 928,384 1,119,337
Available-for-sale Securities $ 1,247,158 $ 1,917,996