Investments (Credit Quality Indicators For Mortgage Loans) (Details) $ in Millions	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 12,217	$ 10,762
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	$ 11,043	$ 9,642
Percentage of total mortgage loans on real estate	90.40%	89.60%
Debt-service coverage ratio	2.31	2.26
Loan-to-value ratio, 65% to 74% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 1,081	$ 1,000
Percentage of total mortgage loans on real estate	8.80%	9.30%
Debt-service coverage ratio	1.84	1.94
Loan-to-value ratio, 75% to 100% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 87	$ 112
Percentage of total mortgage loans on real estate	0.70%	1.00%
Debt-service coverage ratio	1.03	0.97
Loan-To-Value Ratio, Greater Than 100% [Member]
Mortgage Loans Credit Quality [Line Items]
Carrying value of mortgage loans on real estate	$ 6	$ 8
Percentage of total mortgage loans on real estate	0.10%	0.10%
Debt-service coverage ratio	0.77	0.82