Investments (excluding Consolidated Investment Entities) - Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	$ 44,366	$ 44,743
Embedded Derivatives	37	55
Fixed maturities, including securities pledged	48,329	47,394
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	29	35
Securities pledged, Amortized Cost	1,823	1,261
Securities pledged	2,087	1,409
Total equity securities, Amortized Cost	353	229
Equity securities, available-for-sale, at fair value	380	258
Total fixed maturities and equity securities, Amortized Cost	47,737	48,037
Gross Unrealized Capital Gains	4,176	3,143
Gross Unrealized Capital Losses	223	518
Fair Value	51,727	50,717
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	2,047	2,150
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	477	407
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	2	2
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	2,522	2,555
U.S. government agencies and authorities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	223	227
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	52	41
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	0	0
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	275	268
State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	1,856	1,647
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	68	23
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	11	39
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	1,913	1,631
U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	20,857	21,873
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	2,451	1,722
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	50	178
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	23,258	23,417
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities		6
U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	5,628	5,076
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	255	174
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	50	113
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	5,833	5,137
Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	5,241	5,161
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	493	293
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	18	69
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	5,716	5,385
Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	4,974	4,954
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	251	206
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	64	52
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	5,161	5,108
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	10
Residential mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	4,247	4,565
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	274	306
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	34	48
Embedded Derivatives	37	55
Fixed maturities, including securities pledged	4,524	4,878
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	16	25
Residential mortgage-backed securities, Agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	2,990	3,720
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	164	209
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	30	42
Embedded Derivatives	21	32
Fixed maturities, including securities pledged	3,145	3,919
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0
Residential mortgage-backed securities, Non-Agency
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	1,257	845
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	110	97
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	4	6
Embedded Derivatives	16	23
Fixed maturities, including securities pledged	1,379	959
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	16	25
Commercial mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	2,646	2,320
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	69	59
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	11	24
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	2,704	2,355
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	1,488	1,096
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	43	43
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	3	5
Embedded Derivatives	0	0
Fixed maturities, including securities pledged	1,528	1,134
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	3	4
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	49,207	49,069
Fixed maturities, including securities pledged, Gross Unrealized Capital Gains	4,433	3,274
Fixed maturities, including securities pledged,Gross Unrealized Capital Losses	243	530
Embedded Derivatives	37	55
Fixed maturities, including securities pledged	53,434	51,868
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	29	35
Securities pledged, Amortized Cost	1,823	1,261
Securities pledged, Gross Unrealized Capital Gains	284	160
Securities pledged, Gross Unrealized Capital Losses	20	12
Securities pledged	2,087	1,409
Total fixed maturities, less securities pledged, Amortized Cost	47,384	47,808
Total fixed maturities, less securities pledged, Gross Unrealized Capital Gains	4,149	3,114
Total fixed maturities, less securities pledged, Gross Unrealized Capital Losses	223	518
Total fixed maturities, less securities pledged, Fair Value	51,347	50,459
Equity securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	0
Total equity securities, Amortized Cost	353	229
Equity securities, Gross Unrealized Capital Gains	27	29
Equity securities, Gross Unrealized Capital Losses	0	0
Equity securities, available-for-sale, at fair value	380	258
Common stock
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	0
Total equity securities, Amortized Cost	272	152
Equity securities, Gross Unrealized Capital Gains	1	0
Equity securities, Gross Unrealized Capital Losses	0	0
Equity securities, available-for-sale, at fair value	273	152
Preferred stock
Available-for-sale Securities, Including Securities Pledged [Line Items]
Embedded Derivatives	0	0
Total equity securities, Amortized Cost	81	77
Equity securities, Gross Unrealized Capital Gains	26	29
Equity securities, Gross Unrealized Capital Losses	0	0
Equity securities, available-for-sale, at fair value	107	106
Impaired
Available-for-sale Securities, Including Securities Pledged [Line Items]
Net unrealized gains on impaired available-for-sale securities	$ 441	$ 408