Variable Interest Entities - Summary of Investment Funds (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Investment funds	$ 3,390	$ 2,580
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 692	$ 699
Percent of total	100.00%	100.00%
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Private equity
Variable Interest Entity [Line Items]
Investment funds	$ 254	$ 271
Percent of total	36.70%	38.80%
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Real estate and other real assets
Variable Interest Entity [Line Items]
Investment funds	$ 205	$ 161
Percent of total	29.60%	23.00%
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Natural resources
Variable Interest Entity [Line Items]
Investment funds	$ 4	$ 4
Percent of total	0.60%	0.60%
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Hedge funds
Variable Interest Entity [Line Items]
Investment funds	$ 50	$ 61
Percent of total	7.20%	8.70%
Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Credit funds
Variable Interest Entity [Line Items]
Investment funds	$ 179	$ 202
Percent of total	25.90%	28.90%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 2,093	$ 1,310
Percent of total	100.00%	100.00%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Private equity – other | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 680	$ 180
Percent of total	32.50%	13.70%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Real estate and other real assets | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 499	$ 297
Percent of total	23.80%	22.70%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Natural resources | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 95	$ 74
Percent of total	4.50%	5.60%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Hedge funds | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 98	$ 93
Percent of total	4.70%	7.10%
Consolidated Entity Excluding Variable Interest Entities (VIE) | Credit funds | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 272	$ 263
Percent of total	13.00%	20.10%
Variable Interest Entities
Variable Interest Entity [Line Items]
Investment funds	$ 605	$ 571
Percent of total	100.00%	100.00%
Variable Interest Entities | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 564	$ 571
Variable Interest Entities | Real estate and other real assets
Variable Interest Entity [Line Items]
Investment funds	$ 55	$ 22
Percent of total	9.10%	3.80%
Variable Interest Entities | Credit funds
Variable Interest Entity [Line Items]
Investment funds	$ 1	$ 21
Percent of total	0.20%	3.70%
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Private equity
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Real estate and other real assets
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	1 year
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Natural resources
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	1 year
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Hedge funds
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Credit funds
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Private equity – other | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Real estate and other real assets | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Natural resources | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	3 years	4 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Hedge funds | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	11 years	9 years
Minimum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Credit funds | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	2 years
Minimum | Variable Interest Entities | Real estate and other real assets
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	2 years
Minimum | Variable Interest Entities | Credit funds
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	0 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Private equity
Variable Interest Entity [Line Items]
Life of underlying funds	5 years	7 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Real estate and other real assets
Variable Interest Entity [Line Items]
Life of underlying funds	6 years	7 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Natural resources
Variable Interest Entity [Line Items]
Life of underlying funds	0 years	1 year
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Hedge funds
Variable Interest Entity [Line Items]
Life of underlying funds	1 year	3 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) and Related Party | Credit funds
Variable Interest Entity [Line Items]
Life of underlying funds	4 years	5 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Private equity – other | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	9 years	10 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Real estate and other real assets | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	10 years	7 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Natural resources | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	4 years	6 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Hedge funds | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	11 years	9 years
Maximum | Consolidated Entity Excluding Variable Interest Entities (VIE) | Credit funds | Related Party
Variable Interest Entity [Line Items]
Life of underlying funds	3 years	4 years
Maximum | Variable Interest Entities | Real estate and other real assets
Variable Interest Entity [Line Items]
Life of underlying funds	6 years	3 years
Maximum | Variable Interest Entities | Credit funds
Variable Interest Entity [Line Items]
Life of underlying funds	1 year	3 years
A-A Mortgage Opportunities, LP | Related Party
Variable Interest Entity [Line Items]
Investment including loans	$ 459	$ 455
A-A Mortgage Opportunities, LP | Consolidated Entity Excluding Variable Interest Entities (VIE) | Private equity | Related Party
Variable Interest Entity [Line Items]
Investment funds	$ 449	$ 403
Percent of total	21.50%	30.80%
Life of underlying funds	5 years	5 years
MidCap Holdings | Related Party
Variable Interest Entity [Line Items]
Investment including loans	$ 788	$ 766
MidCap Holdings | Variable Interest Entities | Private equity
Variable Interest Entity [Line Items]
Investment funds	$ 549	$ 528
Percent of total	90.70%	92.50%