Fair value measurement - Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	$ 126,959	$ 141,775
Interest-bearing deposits with banks	20,057	17,300
Securities held-to-maturity	56,708	49,224
Other financial assets	1,313	1,009
Liabilities:
Noninterest-bearing deposits	100,498	83,854
Payables to customers and broker-dealers	26,002	25,085
Long-term debt	400	400
Estimated fair value
Assets:
Interest-bearing deposits with the Federal Reserve and other central banks	126,959	141,775
Interest-bearing deposits with banks	20,063	17,310
Federal funds sold and securities purchased under resale agreements	28,497	30,907
Securities held-to-maturity	56,708	49,224
Loans	63,513	53,586
Other financial assets	7,938	7,412
Total assets	303,678	300,214
Liabilities:
Noninterest-bearing deposits	100,498	83,854
Interest-bearing deposits	241,183	257,287
Federal funds purchased and securities sold under repurchase agreements	11,973	11,305
Payables to customers and broker-dealers	26,002	25,085
Borrowings	929	563
Long-term debt	25,741	27,306
Total liabilities	406,326	405,400
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	11,538	4,120
Loans	0	0
Other financial assets	6,752	6,252
Total assets	18,290	10,372
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
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	126,959	141,775
Interest-bearing deposits with banks	20,063	17,310
Federal funds sold and securities purchased under resale agreements	28,497	30,907
Securities held-to-maturity	45,170	45,104
Loans	63,513	53,586
Other financial assets	1,186	1,160
Total assets	285,388	289,842
Liabilities:
Noninterest-bearing deposits	100,498	83,854
Interest-bearing deposits	241,183	257,287
Federal funds purchased and securities sold under repurchase agreements	11,973	11,305
Payables to customers and broker-dealers	26,002	25,085
Borrowings	929	563
Long-term debt	25,741	27,306
Total liabilities	406,326	405,400
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 financial assets	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
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	126,959	141,775
Interest-bearing deposits with banks	20,057	17,300
Federal funds sold and securities purchased under resale agreements	28,497	30,907
Securities held-to-maturity	56,267	47,946
Loans	63,322	55,121
Other financial assets	7,938	7,412
Total assets	303,040	300,461
Liabilities:
Noninterest-bearing deposits	100,498	83,854
Interest-bearing deposits	242,641	257,691
Federal funds purchased and securities sold under repurchase agreements	11,973	11,305
Payables to customers and broker-dealers	26,002	25,085
Borrowings	929	563
Long-term debt	24,643	25,584
Total liabilities	$ 406,686	$ 404,082