Investments - Carrying value for limited partnership interests (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 7,232	$ 8,078
EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,464	6,264
Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,768	1,814
Private equity
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	5,828	6,131
Private equity | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	4,180	4,463
Private equity | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,648	1,668
Other investments
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	287	906
Other investments | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	287	902
Other investments | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	0	4
Real estate
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	1,117	1,041
Real estate | EMA limited partnerships
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	997	899
Real estate | Limited partnership interests
Debt and Equity Securities, FV-NI [Line Items]
Limited partnership interests	$ 120	$ 142