Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2020
Government Of Japan [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,429	$ 813
Available-for-sale Securities	1,430	852
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,012	1,014
Available-for-sale Securities	1,470	1,379
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,226	1,205
Available-for-sale Securities	$ 2,347	$ 2,163