Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2019
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 981,279	$ 913,642
Available-for-sale Securities	1,371,571	1,187,526
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,167,744	1,091,018
Available-for-sale Securities	$ 2,153,434	$ 1,757,087