Derivative instruments - Secured Borrowings (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 69,494	$ 104,451
Securities lending	1,002	718
Total borrowings	70,496	105,169
Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	64,323	99,953
Securities lending	1,002	718
Total borrowings	65,325	100,671
Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	875	1,323
Securities lending	0	0
Total borrowings	875	1,323
30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	4,296	3,175
Securities lending	0	0
Total borrowings	4,296	3,175
U.S. Treasury
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	60,350	94,798
U.S. Treasury | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	60,350	94,788
U.S. Treasury | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	10
U.S. Treasury | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	0
Agency RMBS
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	3,590	5,008
Securities lending	180	160
Agency RMBS | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	2,913	4,234
Securities lending	180	160
Agency RMBS | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	675	774
Securities lending	0	0
Agency RMBS | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	2	0
Securities lending	0	0
Corporate bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	1,728	2,119
Corporate bonds | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	232	266
Corporate bonds | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	64	236
Corporate bonds | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	1,432	1,617
Sovereign debt/sovereign guaranteed
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	1,279	22
Sovereign debt/sovereign guaranteed | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	128	0
Sovereign debt/sovereign guaranteed | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	22
Sovereign debt/sovereign guaranteed | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	1,151	0
State and political subdivisions
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	892	1,281
State and political subdivisions | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	43	38
State and political subdivisions | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	39	166
State and political subdivisions | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	810	1,077
U.S. government agencies
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	610	610
Securities lending	1	19
U.S. government agencies | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	610	594
Securities lending	1	19
U.S. government agencies | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	16
Securities lending	0	0
U.S. government agencies | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	0
Securities lending	0	0
Other debt securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	277	4
Securities lending	49	41
Other debt securities | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	47	2
Securities lending	49	41
Other debt securities | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	44	0
Securities lending	0	0
Other debt securities | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	186	2
Securities lending	0	0
Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	768	609
Securities lending	772	498
Equity securities | Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	31
Securities lending	772	498
Equity securities | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	53	99
Securities lending	0	0
Equity securities | 30 days or more
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	715	479
Securities lending	$ 0	$ 0