Investments (excluding Consolidated Investment Entities) - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 6,042	$ 12,117
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	230	513
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	57	33
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	3
More Than Twelve Months Below Amortized Cost, Fair Value	476	431
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	13	14
Total Fair Value	6,575	12,581
Total Unrealized Capital Losses	$ 243	$ 530
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	97.30%	96.90%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	$ 166	$ 209
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	2	2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	15	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	0	0
Total Fair Value	181	209
Total Unrealized Capital Losses	2	2
State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	356	945
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	9	38
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	6	2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	35	49
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	2	1
Total Fair Value	397	996
Total Unrealized Capital Losses	11	39
U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,399	4,568
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	47	175
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	8	14
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	114	112
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	3	3
Total Fair Value	1,521	4,694
Total Unrealized Capital Losses	50	178
U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	1,068	1,596
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	46	109
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	0	10
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	1
More Than Twelve Months Below Amortized Cost, Fair Value	84	87
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	4	3
Total Fair Value	1,152	1,693
Total Unrealized Capital Losses	50	113
Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	463	1,274
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	17	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	6	6
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	2
More Than Twelve Months Below Amortized Cost, Fair Value	26	139
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	1	4
Total Fair Value	495	1,419
Total Unrealized Capital Losses	18	69
Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	493	1,026
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	64	52
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	9	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	8	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	0	0
Total Fair Value	510	1,026
Total Unrealized Capital Losses	64	52
Residential mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	967	1,389
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	32	47
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	6	1
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	81	21
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	2	1
Total Fair Value	1,054	1,411
Total Unrealized Capital Losses	34	48
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	756	680
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	10	22
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	18	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	86	23
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	1	2
Total Fair Value	860	703
Total Unrealized Capital Losses	11	24
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Fair Value	374	430
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	3	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Fair Value	4	0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Fair Value	27	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	0	0
Total Fair Value	405	430
Total Unrealized Capital Losses	$ 3	$ 5