Investments - Carrying value for limited partnership interests (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 7,609	$ 8,078
EMA
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,785	6,264
Fair Value
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,824	1,814
Private equity
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	6,125	6,131
Private equity | EMA
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	4,417	4,463
Private equity | Fair Value
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,708	1,668
Real estate
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,074	1,041
Real estate | EMA
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	958	899
Real estate | Fair Value
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	116	142
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	410	906
Other investments | EMA
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	410	902
Other investments | Fair Value
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 0	$ 4