Investments (excluding Consolidated Investment Entities) - Unrealized Capital Losses (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	$ 8,318	$ 6,042
Six months or less below amortized cost, Unrealized Capital Loss	167	230
More than six months and twelve months or less below amortized cost, Fair Value	8,188	57
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	366	0
More than twelve months below amortized cost, Fair Value	3,285	476
More than twelve months below amortized cost, Unrealized Capital Loss	323	13
Total, Fair Value	19,791	6,575
Total Unrealized Capital Losses	$ 856	$ 243
Average market value of fixed maturities with unrealized capital losses aged more than twelve months	91.00%	97.30%
U.S. Treasuries
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	$ 21	$ 166
Six months or less below amortized cost, Unrealized Capital Loss	0	2
More than six months and twelve months or less below amortized cost, Fair Value	81	0
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	3	0
More than twelve months below amortized cost, Fair Value	54	15
More than twelve months below amortized cost, Unrealized Capital Loss	2	0
Total, Fair Value	156	181
Total Unrealized Capital Losses	5	2
State, municipalities and political subdivisions
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	467	356
Six months or less below amortized cost, Unrealized Capital Loss	9	9
More than six months and twelve months or less below amortized cost, Fair Value	307	6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	13	0
More than twelve months below amortized cost, Fair Value	220	35
More than twelve months below amortized cost, Unrealized Capital Loss	16	2
Total, Fair Value	994	397
Total Unrealized Capital Losses	38	11
U.S. corporate public securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	2,727	1,399
Six months or less below amortized cost, Unrealized Capital Loss	53	47
More than six months and twelve months or less below amortized cost, Fair Value	3,084	8
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	139	0
More than twelve months below amortized cost, Fair Value	732	114
More than twelve months below amortized cost, Unrealized Capital Loss	79	3
Total, Fair Value	6,543	1,521
Total Unrealized Capital Losses	271	50
U.S. corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	1,010	1,068
Six months or less below amortized cost, Unrealized Capital Loss	11	46
More than six months and twelve months or less below amortized cost, Fair Value	1,268	0
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	44	0
More than twelve months below amortized cost, Fair Value	846	84
More than twelve months below amortized cost, Unrealized Capital Loss	94	4
Total, Fair Value	3,124	1,152
Total Unrealized Capital Losses	149	50
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	987	463
Six months or less below amortized cost, Unrealized Capital Loss	17	17
More than six months and twelve months or less below amortized cost, Fair Value	1,223	6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	62	0
More than twelve months below amortized cost, Fair Value	194	26
More than twelve months below amortized cost, Unrealized Capital Loss	25	1
Total, Fair Value	2,404	495
Total Unrealized Capital Losses	104	18
Foreign corporate private securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	977	493
Six months or less below amortized cost, Unrealized Capital Loss	52	64
More than six months and twelve months or less below amortized cost, Fair Value	805	9
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	33	0
More than twelve months below amortized cost, Fair Value	336	8
More than twelve months below amortized cost, Unrealized Capital Loss	34	0
Total, Fair Value	2,118	510
Total Unrealized Capital Losses	119	64
Residential mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	384	967
Six months or less below amortized cost, Unrealized Capital Loss	6	32
More than six months and twelve months or less below amortized cost, Fair Value	432	6
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	25	0
More than twelve months below amortized cost, Fair Value	537	81
More than twelve months below amortized cost, Unrealized Capital Loss	49	2
Total, Fair Value	1,353	1,054
Total Unrealized Capital Losses	80	34
Commercial mortgage-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	1,031	756
Six months or less below amortized cost, Unrealized Capital Loss	14	10
More than six months and twelve months or less below amortized cost, Fair Value	749	18
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	41	0
More than twelve months below amortized cost, Fair Value	298	86
More than twelve months below amortized cost, Unrealized Capital Loss	22	1
Total, Fair Value	2,078	860
Total Unrealized Capital Losses	77	11
Other asset-backed
Available-for-sale Securities, Including Securities Pledged [Line Items]
Six months or less below amortized cost, Fair Value	714	374
Six months or less below amortized cost, Unrealized Capital Loss	5	3
More than six months and twelve months or less below amortized cost, Fair Value	239	4
More than six months and twelve months or less below amortized cost, Unrealized Capital Losses	6	0
More than twelve months below amortized cost, Fair Value	68	27
More than twelve months below amortized cost, Unrealized Capital Loss	2	0
Total, Fair Value	1,021	405
Total Unrealized Capital Losses	$ 13	$ 3