Derivative instruments - Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Asset
Derivatives subject to netting arrangements, Gross assets recognized	$ 7,234	$ 7,264
Total derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	5,238	5,819
Derivatives subject to netting arrangements, Net assets recognized in the balance sheet	1,996	1,445
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	383	251
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Derivative assets subject to netting arrangements, Net amount	1,613	1,194
Total derivatives not subject to netting arrangements	2,209	1,797
Total derivatives, Gross assets recognized	9,443	9,061
Fair value after effect of master netting agreements	4,205	3,242
Total derivatives, Net amount	3,822	2,991
Reverse repurchase agreements, Gross assets recognized	72,507	112,355
Reverse repurchase agreements, Gross amounts offset in the balance sheet	54,629	93,794
Reverse repurchase agreements, Net assets recognized on the balance sheet	17,878	18,561
Reverse repurchase agreements, Gross amounts not offset in the balance sheet, Financial instruments	17,852	18,554
Reverse repurchase agreements, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Reverse repurchase agreements, Net amount	26	7
Securities borrowing, Gross assets recognized	11,769	11,621
Securities borrowing, Net assets recognized on the balance sheet	11,769	11,621
Securities borrowing, Gross amounts not offset in the balance sheet, Financial instruments	11,216	11,278
Securities borrowing, Net amount	553	343
Total, Gross assets recognized	93,719	133,037
Total, Gross amounts offset in the balance sheet	59,867	99,613
Total, Net assets recognized on the balance sheet	33,852	33,424
Total, Gross amounts not offset in the balance sheet, Financial instruments	29,451	30,083
Total, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Total, Net amount	4,401	3,341
Derivative Liability
Derivatives subject to netting arrangements, Gross liabilities recognized	9,337	8,428
Derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	6,183	5,415
Derivatives subject to netting arrangements, Net liabilities recognized in the balance sheet	3,154	3,013
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	2,701	1,615
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral pledged	0	0
Derivative liabilities subject to netting arrangements, Net amount	453	1,398
Total derivatives not subject to netting arrangements	439	778
Total derivatives, Gross liabilities recognized	9,776	9,206
Fair value after effect of master netting agreements	3,593	3,791
Total derivatives, Net amount	892	2,176
Repurchase agreements, Gross liabilities recognized	69,494	104,451
Repurchase agreements, Gross amounts offset in the balance sheet	54,629	93,794
Repurchase agreements, Net liabilities recognized on the balance sheet	14,865	10,657
Repurchase agreements, Gross amounts not offset in the balance sheet, Financial instruments	14,863	10,657
Repurchase agreements, Gross amounts not offset in the balance sheet, Cash collateral pledged	1	0
Repurchase agreements, Net amount	1	0
Securities lending, Gross liabilities recognized	1,002	718
Securities lending, Net liabilities recognized on the balance sheet	1,002	718
Securities lending, Gross amounts not offset in the balance sheet, Financial instruments	961	694
Securities lending, Net amount	41	24
Total, Gross liabilities recognized	80,272	114,375
Total, Gross amounts offset in the balance sheet	60,812	99,209
Total, Net liabilities recognized on the balance sheet	19,460	15,166
Total, Gross amounts not offset in the balance sheet, Financial instruments	18,525	12,966
Total, Gross amounts not offset in the balance sheet, Cash collateral pledged	1	0
Total, Net amount	934	2,200
Interest rate contracts
Derivative Asset
Derivatives subject to netting arrangements, Gross assets recognized	3,183	2,394
Total derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	2,133	1,792
Derivatives subject to netting arrangements, Net assets recognized in the balance sheet	1,050	602
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	350	207
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Derivative assets subject to netting arrangements, Net amount	700	395
Derivative Liability
Derivatives subject to netting arrangements, Gross liabilities recognized	5,035	3,550
Derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	2,551	1,986
Derivatives subject to netting arrangements, Net liabilities recognized in the balance sheet	2,484	1,564
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	2,477	1,539
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral pledged	0	0
Derivative liabilities subject to netting arrangements, Net amount	7	25
Foreign exchange contracts
Derivative Asset
Derivatives subject to netting arrangements, Gross assets recognized	4,043	4,861
Total derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	3,102	4,021
Derivatives subject to netting arrangements, Net assets recognized in the balance sheet	941	840
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	33	44
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Derivative assets subject to netting arrangements, Net amount	908	796
Derivative Liability
Derivatives subject to netting arrangements, Gross liabilities recognized	4,294	4,873
Derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	3,631	3,428
Derivatives subject to netting arrangements, Net liabilities recognized in the balance sheet	663	1,445
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	224	74
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral pledged	0	0
Derivative liabilities subject to netting arrangements, Net amount	439	1,371
Equity and other contracts
Derivative Asset
Derivatives subject to netting arrangements, Gross assets recognized	8	9
Total derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	3	6
Derivatives subject to netting arrangements, Net assets recognized in the balance sheet	5	3
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	0	0
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral received	0	0
Derivative assets subject to netting arrangements, Net amount	5	3
Derivative Liability
Derivatives subject to netting arrangements, Gross liabilities recognized	8	5
Derivatives subject to netting arrangements, Gross amounts offset in the balance sheet	1	1
Derivatives subject to netting arrangements, Net liabilities recognized in the balance sheet	7	4
Total derivatives, Gross amounts not offset in the balance sheet, Financial instruments	0	2
Total derivatives, Gross amounts not offset in the balance sheet, Cash collateral pledged	0	0
Derivative liabilities subject to netting arrangements, Net amount	$ 7	$ 2