Investments (Credit Quality Indicators For Mortgage Loans) (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 10,762	$ 9,889
Percentage of total mortgage loans on real estate	100.00%	100.00%
Loan-to-value ratio, less than 65% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 9,642	$ 8,709
Percentage of total mortgage loans on real estate	89.60%	88.00%
Debt-service coverage ratio	2.26	2.16
Loan-to-value ratio, 65% to 74% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 1,000	$ 1,009
Percentage of total mortgage loans on real estate	9.30%	10.20%
Debt-service coverage ratio	1.94	1.87
Loan-to-value ratio, 75% to 100% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 112	$ 166
Percentage of total mortgage loans on real estate	1.00%	1.70%
Debt-service coverage ratio	0.97	0.82
Loan-To-Value Ratio, Greater Than 100% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 8	$ 5
Percentage of total mortgage loans on real estate	0.10%	0.10%
Debt-service coverage ratio	0.82	1.04