Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,252,180	$ 18,924,711
Available-for-sale Securities	22,341,661	19,619,084
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	886,518	832,764
Available-for-sale Securities	1,228,391	1,126,433
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,060,922	1,004,261
Available-for-sale Securities	$ 1,785,573	$ 1,612,957