Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 24,216,876	$ 24,006,407
Available-for-sale Securities	25,077,338	23,982,119
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,128,373	913,642
Available-for-sale Securities	1,955,925	1,187,526
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	946,731	1,091,018
Available-for-sale Securities	$ 1,287,150	$ 1,757,087