Investments (excluding Consolidated Investment Entities) - Unrealized Capital Losses 1 (Details) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	$ 230	$ 513
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	3
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	13	14
Total Unrealized Capital Losses	243	530
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	2	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, Unrealized Capital Losses	0	0
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, Unrealized Capital Losses	9	38
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	2	1
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, Unrealized Capital Losses	47	175
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	3	3
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, Unrealized Capital Losses	46	109
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	1
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	4	3
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, Unrealized Capital Losses	17	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	2
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	1	4
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, Unrealized Capital Losses	64	52
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	0	0
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, Unrealized Capital Losses	32	47
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	2	1
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, Unrealized Capital Losses	10	22
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	1	2
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, Unrealized Capital Losses	3	5
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	0	0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	0	0
Total Unrealized Capital Losses	3	5
Fair value decline below amortized cost less than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Amortized Cost	6,126	12,536
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	$ 148	$ 466
Six Months or Less Below Amortized Cost, Number of Securities | security	1,098	1,694
More Than Six Months and Twelve Months or Less Below Amortized Cost, Amortized Cost	$ 48	$ 45
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 1	$ 2
More Than Six Months and Twelve Months or Less Below Amortized Cost, Number of Securities | security	14	13
More Than Twelve Months Below Amortized Cost, Amortized Cost	$ 448	$ 335
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	$ 12	$ 9
More Than Twelve Months Below Amortized Cost, Number of Securities | security	87	38
Total Amortized Cost	$ 6,622	$ 12,916
Total Unrealized Capital Losses	$ 161	$ 477
Number of Securities | security	1,199	1,745
Fair value decline below amortized cost less than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 183	$ 211
Total Unrealized Capital Losses	$ 2	$ 2
Number of Securities | security	29	25
Fair value decline below amortized cost less than 20% | State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 408	$ 1,034
Total Unrealized Capital Losses	$ 11	$ 39
Number of Securities | security	103	198
Fair value decline below amortized cost less than 20% | U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1,553	$ 4,811
Total Unrealized Capital Losses	$ 45	$ 163
Number of Securities | security	232	547
Fair value decline below amortized cost less than 20% | U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1,129	$ 1,699
Total Unrealized Capital Losses	$ 28	$ 84
Number of Securities | security	73	111
Fair value decline below amortized cost less than 20% | Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 506	$ 1,471
Total Unrealized Capital Losses	$ 16	$ 64
Number of Securities | security	84	186
Fair value decline below amortized cost less than 20% | Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 490	$ 1,078
Total Unrealized Capital Losses	$ 16	$ 52
Number of Securities | security	35	64
Fair value decline below amortized cost less than 20% | Residential mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1,075	$ 1,452
Total Unrealized Capital Losses	$ 29	$ 45
Number of Securities | security	334	365
Fair value decline below amortized cost less than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 871	$ 727
Total Unrealized Capital Losses	$ 11	$ 24
Number of Securities | security	164	124
Fair value decline below amortized cost less than 20% | Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 407	$ 433
Total Unrealized Capital Losses	$ 3	$ 4
Number of Securities | security	145	125
Fair value decline below amortized cost greater than 20%
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six Months or Less Below Amortized Cost, Amortized Cost	$ 196	$ 195
Six Months or Less Below Amortized Cost, Unrealized Capital Losses	$ 82	$ 53
Six Months or Less Below Amortized Cost, Number of Securities | security	38	63
More Than Six Months and Twelve Months or Less Below Amortized Cost, Amortized Cost	$ 0	$ 0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Unrealized Capital Loss	$ 0	$ 0
More Than Six Months and Twelve Months or Less Below Amortized Cost, Number of Securities | security	0	0
More Than Twelve Months Below Amortized Cost, Amortized Cost	$ 0	$ 0
More Than Twelve Months Below Amortized Cost, Unrealized Capital Losses	$ 0	$ 0
More Than Twelve Months Below Amortized Cost, Number of Securities | security	0	1
Total Amortized Cost	$ 196	$ 195
Total Unrealized Capital Losses	$ 82	$ 53
Number of Securities | security	38	64
Fair value decline below amortized cost greater than 20% | U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Number of Securities | security	0	0
Fair value decline below amortized cost greater than 20% | State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 1
Total Unrealized Capital Losses	$ 0	$ 0
Number of Securities | security	0	1
Fair value decline below amortized cost greater than 20% | U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 18	$ 61
Total Unrealized Capital Losses	$ 5	$ 15
Number of Securities | security	2	17
Fair value decline below amortized cost greater than 20% | U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 73	$ 107
Total Unrealized Capital Losses	$ 22	$ 29
Number of Securities | security	2	3
Fair value decline below amortized cost greater than 20% | Foreign corporate public securities and foreign governments
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 7	$ 17
Total Unrealized Capital Losses	$ 2	$ 5
Number of Securities | security	1	10
Fair value decline below amortized cost greater than 20% | Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 84	$ 0
Total Unrealized Capital Losses	$ 48	$ 0
Number of Securities | security	6	2
Fair value decline below amortized cost greater than 20% | Residential mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 13	$ 7
Total Unrealized Capital Losses	$ 5	$ 3
Number of Securities | security	25	28
Fair value decline below amortized cost greater than 20% | Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 0	$ 0
Total Unrealized Capital Losses	$ 0	$ 0
Number of Securities | security	0	2
Fair value decline below amortized cost greater than 20% | Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Total Amortized Cost	$ 1	$ 2
Total Unrealized Capital Losses	$ 0	$ 1
Number of Securities | security	2	1