Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Financial Assets
Cash and due from banks	$ 20,540	$ 15,705	$ 23,664	$ 11,147
Investment securities held-to-maturity	44,018	42,991
Loans	274,811	269,394
Financial Liabilities
Deposits	342,589	334,590
Long-term debt	34,515	33,323
Carrying Amount [Member]
Financial Assets
Cash and due from banks	20,540	15,705
Federal funds sold and securities purchased under resale agreements	78	138
Investment securities held-to-maturity	44,018	42,991
Loans held for sale	3	4
Loans	274,811	269,394
Other financial instruments	2,449	2,362
Financial Liabilities
Deposits	342,589	334,590
Short-term borrowings	14,641	12,891
Long-term debt	34,515	33,323
Other liabilities	1,659	1,702
Fair Value [Member]
Financial Assets
Cash and due from banks	20,540	15,705
Federal funds sold and securities purchased under resale agreements	78	138
Investment securities held-to-maturity	43,758	42,435
Loans held for sale	3	4
Loans	278,946	273,422
Other financial instruments	2,456	2,369
Financial Liabilities
Deposits	342,411	334,361
Short-term borrowings	14,430	12,706
Long-term debt	34,842	33,678
Other liabilities	1,659	1,702
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	20,540	15,705
Investment securities held-to-maturity	4,668	4,605
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	78	138
Investment securities held-to-maturity	39,074	37,810
Other financial instruments	1,037	920
Financial Liabilities
Deposits	342,411	334,361
Short-term borrowings	14,430	12,706
Long-term debt	34,842	33,678
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	16	20
Loans held for sale	3	4
Loans	278,946	273,422
Other financial instruments	1,419	1,449
Financial Liabilities
Other liabilities	$ 1,659	$ 1,702