Investments - Credit Risk Profile, Commercial Mortgage Loans (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	$ 1,684	$ 1,666
Weighted Average [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Debt service coverage ratio, commercial mortgage loans	2.1	1.95
Loan-to-value ratio, commercial mortgage loans	57.00%	57.00%
Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	$ 363	$ 350
Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	678	593
Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	524	638
Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	119	64
Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	21
Debt Service Coverage Ratio 1.30x Or Greater [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	1,563	1,476
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	304	335
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	651	517
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	524	624
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	84	0
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.30x Or Greater [Member] | Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.20x To 1.29x [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	41	75
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	14	15
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	27	46
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	14
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.20x To 1.29x [Member] | Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.10x To 1.19x [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	29
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	29
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.10x To 1.19x [Member] | Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.00x To 1.09x [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	30
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	30
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio 1.00x To 1.09x [Member] | Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio Less Than 1.00x [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	80	56
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio Below 50 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	45	0
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio 50 Percent To 59 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio 60 Percent To 69 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio 70 Percent To 79 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	35	35
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio 80 Percent To 89 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	0	0
Debt Service Coverage Ratio Less Than 1.00x [Member] | Loan To Value Ratio 90 Percent To 100 Percent [Member]
Schedule Of Financing Receivable Recorded Investment Credit Quality Indicator [Line Items]
Commercial mortgage loan	$ 0	$ 21