Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 19,281,539	$ 18,924,711
Available-for-sale Securities	20,161,453	19,619,084
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,020,682	832,764
Available-for-sale Securities	1,644,422	1,126,433
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	852,739	1,004,261
Available-for-sale Securities	$ 1,157,169	$ 1,612,957