Fair value measurement - Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets:
Securities held-to-maturity	$ 47,458	$ 34,805
Liabilities:
Long-term debt	400	387
Estimated fair value
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	106,185	95,042
Interest-bearing deposits with banks	19,037	14,832
Federal funds sold and securities purchased under resale agreements	29,647	30,182
Securities held-to-maturity	47,458	34,805
Loans	54,475	54,194
Other assets, fair value	5,225	6,063
Total assets	262,027	235,118
Liabilities:
Noninterest-bearing deposits	79,470	57,630
Interest-bearing deposits	216,499	200,846
Federal funds purchased and securities sold under repurchase agreements	15,907	11,401
Payables to customers and broker-dealers	23,514	18,758
Commercial paper	671	3,959
Borrowings	607	917
Long-term debt	27,457	27,858
Total liabilities	364,125	321,369
Estimated fair value | Level 1
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	0	0
Interest-bearing deposits with banks	0	0
Federal funds sold and securities purchased under resale agreements	0	0
Securities held-to-maturity	4,415	4,630
Loans	0	0
Other assets, fair value	4,104	4,830
Total assets	8,519	9,460
Liabilities:
Noninterest-bearing deposits	0	0
Interest-bearing deposits	0	0
Federal funds purchased and securities sold under repurchase agreements	0	0
Payables to customers and broker-dealers	0	0
Commercial paper	0	0
Borrowings	0	0
Long-term debt	0	0
Total liabilities	0	0
Estimated fair value | Level 2
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	106,185	95,042
Interest-bearing deposits with banks	19,037	14,832
Federal funds sold and securities purchased under resale agreements	29,647	30,182
Securities held-to-maturity	43,043	30,175
Loans	54,475	54,194
Other assets, fair value	1,121	1,233
Total assets	253,508	225,658
Liabilities:
Noninterest-bearing deposits	79,470	57,630
Interest-bearing deposits	216,499	200,846
Federal funds purchased and securities sold under repurchase agreements	15,907	11,401
Payables to customers and broker-dealers	23,514	18,758
Commercial paper	671	3,959
Borrowings	607	917
Long-term debt	27,457	27,858
Total liabilities	364,125	321,369
Estimated fair value | Level 3
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	0	0
Interest-bearing deposits with banks	0	0
Federal funds sold and securities purchased under resale agreements	0	0
Securities held-to-maturity	0	0
Loans	0	0
Other assets, fair value	0	0
Total assets	0	0
Liabilities:
Noninterest-bearing deposits	0	0
Interest-bearing deposits	0	0
Federal funds purchased and securities sold under repurchase agreements	0	0
Payables to customers and broker-dealers	0	0
Commercial paper	0	0
Borrowings	0	0
Long-term debt	0	0
Total liabilities	0	0
Carrying amount
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	106,185	95,042
Interest-bearing deposits with banks	19,027	14,811
Federal funds sold and securities purchased under resale agreements	29,647	30,182
Securities held-to-maturity	46,096	34,483
Loans	54,088	53,718
Other assets, fair value	5,225	6,063
Total assets	260,268	234,299
Liabilities:
Noninterest-bearing deposits	79,470	57,630
Interest-bearing deposits	216,842	201,836
Federal funds purchased and securities sold under repurchase agreements	15,907	11,401
Payables to customers and broker-dealers	23,514	18,758
Commercial paper	671	3,959
Borrowings	607	917
Long-term debt	25,721	27,114
Total liabilities	$ 362,732	$ 321,615