Investments - Carrying value for limited partnership interests (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 7,087	$ 8,078
EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,370	6,264
Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,717	1,814
Private equity
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,781	6,131
Private equity | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	4,204	4,463
Private equity | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,577	1,668
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	216	906
Other investments | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	212	902
Other investments | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	4	4
Real estate
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,090	1,041
Real estate | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	954	899
Real estate | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 136	$ 142