Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Financial Assets
Cash and due from banks	$ 10,981	$ 11,147	$ 14,072	$ 10,654
Investment securities held-to-maturity	42,113	43,590
Financial Liabilities
Deposits	306,348	300,400
Long-term debt	34,872	32,078
Carrying Amount [Member]
Financial Assets
Cash and due from banks	10,981	11,147
Federal funds sold and securities purchased under resale agreements	130	169
Investment securities held-to-maturity	42,113	43,590
Loans held for sale	12	74
Loans	260,669	256,986
Other financial instruments	2,320	2,311
Financial Liabilities
Deposits	306,348	300,400
Short-term borrowings	22,967	27,110
Long-term debt	34,872	32,078
Other Liabilities	1,305	1,353
Fair Value [Member]
Financial Assets
Cash and due from banks	10,981	11,147
Federal funds sold and securities purchased under resale agreements	130	169
Investment securities held-to-maturity	42,521	43,493
Loans held for sale	12	74
Loans	265,791	259,823
Other financial instruments	2,325	2,319
Financial Liabilities
Deposits	306,264	300,225
Short-term borrowings	22,749	26,782
Long-term debt	35,452	32,412
Other Liabilities	1,305	1,353
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	10,981	11,147
Investment securities held-to-maturity	2,330	2,275
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	130	169
Investment securities held-to-maturity	40,113	41,138
Other financial instruments	920	921
Financial Liabilities
Deposits	306,264	300,225
Short-term borrowings	22,749	26,782
Long-term debt	35,452	32,412
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	78	80
Loans held for sale	12	74
Loans	265,791	259,823
Other financial instruments	1,405	1,398
Financial Liabilities
Other Liabilities	$ 1,305	$ 1,353