Investments (Exposure To Concentrations Of Credit Risk) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Mar. 31, 2020
Government Of Japan [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 1,660	$ 813
Available-for-sale Securities	1,672	852
Canadian Province Of Quebec Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,132	1,205
Available-for-sale Securities	1,986	2,163
Canadian Province Of Ontario Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	941	1,014
Available-for-sale Securities	$ 1,289	$ 1,379