Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,418,741	$ 18,924,711
Available-for-sale Securities	22,535,830	19,619,084
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	932,872	832,764
Available-for-sale Securities	1,231,201	1,126,433
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,115,505	1,004,261
Available-for-sale Securities	$ 1,766,749	$ 1,612,957