Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Jun. 30, 2019
Available-for-sale Securities, Amortized Cost Basis	$ 26,055,078	$ 24,006,407
Available-for-sale Securities	27,635,431	23,982,119
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	978,025	913,642
Available-for-sale Securities	1,361,749	1,187,526
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,168,749	1,091,018
Available-for-sale Securities	$ 2,115,321	$ 1,757,087