Investments - Carrying value for limited partnership interests (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 6,367	$ 4,563
EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,004	3,501
Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,363	1,062
Private equity
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,230	3,655
Private equity | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	3,935	2,667
Private equity | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,295	988
Real estate
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	884	697
Real estate | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	816	623
Real estate | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	68	74
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	253	211
Other investments | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	253	211
Other investments | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 0	$ 0