Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 24,271,467	$ 21,966,803
Available-for-sale Securities	24,449,498	23,210,968
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	951,186	939,837
Available-for-sale Securities	1,225,712	1,282,944
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,158,326	1,119,337
Available-for-sale Securities	$ 1,833,972	$ 1,917,996