Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 18,924,711	$ 17,575,507
Available-for-sale Securities	19,619,084	17,708,156
Canadian Province Of Ontario Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,004,261	943,484
Available-for-sale Securities	1,612,957	1,525,903
Canadian Province Of Quebec Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	832,764	864,444
Available-for-sale Securities	$ 1,126,433	$ 1,199,080