Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Financial Assets
Cash and due from banks	$ 28,964	$ 15,705	$ 14,038	$ 11,147
Investment securities held-to-maturity	43,659	42,991
Loans	273,427	269,394
Financial Liabilities
Deposits	347,262	334,590
Long-term debt	37,814	33,323
Carrying Amount [Member]
Financial Assets
Cash and due from banks	28,964	15,705
Federal funds sold and securities purchased under resale agreements	76	138
Investment securities held-to-maturity	43,659	42,991
Loans held for sale	5	4
Loans	273,427	269,394
Other financial instruments	2,411	2,362
Financial Liabilities
Deposits	347,262	334,590
Short-term borrowings	13,226	12,891
Long-term debt	37,814	33,323
Other Liabilities	1,590	1,702
Fair Value [Member]
Financial Assets
Cash and due from banks	28,964	15,705
Federal funds sold and securities purchased under resale agreements	76	138
Investment securities held-to-maturity	43,384	42,435
Loans held for sale	5	4
Loans	278,736	273,422
Other financial instruments	2,419	2,369
Financial Liabilities
Deposits	347,043	334,361
Short-term borrowings	13,052	12,706
Long-term debt	38,060	33,678
Other Liabilities	1,590	1,702
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	28,964	15,705
Investment securities held-to-maturity	4,893	4,605
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	76	138
Investment securities held-to-maturity	38,472	37,810
Other financial instruments	992	920
Financial Liabilities
Deposits	347,043	334,361
Short-term borrowings	13,052	12,706
Long-term debt	38,060	33,678
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	19	20
Loans held for sale	5	4
Loans	278,736	273,422
Other financial instruments	1,427	1,449
Financial Liabilities
Other Liabilities	$ 1,590	$ 1,702