Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,966,803	$ 18,924,711
Available-for-sale Securities	23,210,968	19,619,084
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,119,337	1,004,261
Available-for-sale Securities	1,917,996	1,612,957
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	939,837	832,764
Available-for-sale Securities	$ 1,282,944	$ 1,126,433