Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Financial Assets
Cash and due from banks	$ 23,664	$ 11,147	$ 10,450	$ 10,654
Investment securities held-to-maturity	42,873	43,590
Financial Liabilities
Deposits	334,595	300,400
Long-term debt	37,978	32,078
Carrying Amount [Member]
Financial Assets
Cash and due from banks	23,664	11,147
Federal funds sold and securities purchased under resale agreements	209	169
Investment securities held-to-maturity	42,873	43,590
Loans held for sale	3	74
Loans	267,492	256,986
Other financial instruments	2,332	2,311
Financial Liabilities
Deposits	334,595	300,400
Short-term borrowings	14,587	27,110
Long-term debt	37,978	32,078
Other Liabilities	1,634	1,353
Fair Value [Member]
Financial Assets
Cash and due from banks	23,664	11,147
Federal funds sold and securities purchased under resale agreements	209	169
Investment securities held-to-maturity	43,406	43,493
Loans held for sale	3	74
Loans	273,136	259,823
Other financial instruments	2,340	2,319
Financial Liabilities
Deposits	334,522	300,225
Short-term borrowings	14,444	26,782
Long-term debt	38,847	32,412
Other Liabilities	1,634	1,353
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	23,664	11,147
Investment securities held-to-maturity	4,256	2,275
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	209	169
Investment securities held-to-maturity	39,076	41,138
Other financial instruments	920	921
Financial Liabilities
Deposits	334,522	300,225
Short-term borrowings	14,444	26,782
Long-term debt	38,847	32,412
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	74	80
Loans held for sale	3	74
Loans	273,136	259,823
Other financial instruments	1,420	1,398
Financial Liabilities
Other Liabilities	$ 1,634	$ 1,353