Fair Values - Fund Interests Measured Based on Net Asset Value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Measured Based on Net Asset Value
Carrying Value	$ 3,093	$ 2,582
Commitment	527	481
Private equity
Measured Based on Net Asset Value
Carrying Value	1,545	1,374
Commitment	368	316
Carrying Value of Nonredeemable Funds by Contractual Maturity
Less than 5 years	802
5-10 years	720
Over 10 years	23
Total	1,545
Real estate
Measured Based on Net Asset Value
Carrying Value	1,452	1,105
Commitment	155	161
Carrying Value of Nonredeemable Funds by Contractual Maturity
Less than 5 years	634
5-10 years	740
Over 10 years	78
Total	1,452
Hedge
Measured Based on Net Asset Value
Carrying Value	96	103
Commitment	$ 4	$ 4