Fair Values - Fund Interests Measured Based on Net Asset Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Measured Based on Net Asset Value
Carrying Value	$ 3,106	$ 2,582
Commitment	520	481
Private equity
Measured Based on Net Asset Value
Carrying Value	1,542	1,374
Commitment	363	316
Carrying Value of Nonredeemable Funds by Contractual Maturity
Less than 5 years	765
5-10 years	753
Over 10 years	24
Total	1,542
Real estate
Measured Based on Net Asset Value
Carrying Value	1,469	1,105
Commitment	153	161
Carrying Value of Nonredeemable Funds by Contractual Maturity
Less than 5 years	1,151
5-10 years	230
Over 10 years	88
Total	1,469
Hedge
Measured Based on Net Asset Value
Carrying Value	95	103
Commitment	$ 4	$ 4