Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Financial Assets
Cash and due from banks	$ 63,904	$ 62,580	$ 44,047	$ 22,405
Loans	291,816	290,393
Financial Liabilities
Long-term debt	35,671	41,297
Carrying Amount [Member]
Financial Assets
Cash and due from banks	63,904	62,580
Federal funds sold and securities purchased under resale agreements	440	377
Loans held for sale	15	237
Loans	291,816	290,393
Other	1,472	1,772
Financial Liabilities
Time deposits	22,879	30,694
Short-term borrowings	13,990	10,009
Long-term debt	35,671	41,297
Other	3,972	4,052
Fair Value [Member]
Financial Assets
Cash and due from banks	63,904	62,580
Federal funds sold and securities purchased under resale agreements	440	377
Loans held for sale	15	237
Loans	298,764	300,419
Other	1,472	1,772
Financial Liabilities
Time deposits	22,946	30,864
Short-term borrowings	13,894	9,956
Long-term debt	36,262	42,485
Other	3,972	4,052
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	63,904	62,580
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	440	377
Other	630	731
Financial Liabilities
Time deposits	22,946	30,864
Short-term borrowings	13,894	9,956
Long-term debt	36,262	42,485
Other	1,114	1,234
Fair Value [Member] | Level 3 [Member]
Financial Assets
Loans held for sale	15	237
Loans	298,764	300,419
Other	842	1,041
Financial Liabilities
Other	$ 2,858	$ 2,818