Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 24,006,407	$ 21,966,803
Available-for-sale Securities	23,982,119	23,210,968
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,091,018	1,119,337
Available-for-sale Securities	1,757,087	1,917,996
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	913,642	939,837
Available-for-sale Securities	$ 1,187,526	$ 1,282,944