Investments - Carrying value for limited partnership interests (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 6,941	$ 8,078
EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,276	6,264
Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,665	1,814
Private equity
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,575	6,131
Private equity | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	4,043	4,463
Private equity | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,532	1,668
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	254	906
Other investments | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	251	902
Other investments | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	3	4
Real estate
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,112	1,041
Real estate | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	982	899
Real estate | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 130	$ 142