Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 18,762,648	$ 17,575,507
Available-for-sale Securities	20,164,848	17,708,156
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,021,325	943,484
Available-for-sale Securities	1,877,335	1,525,903
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	843,239	864,444
Available-for-sale Securities	$ 1,254,701	$ 1,199,080