Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Financial Assets
Cash and due from banks	$ 14,038	$ 11,147	$ 17,925	$ 10,654
Investment securities held-to-maturity	42,030	43,590
Financial Liabilities
Deposits	317,590	300,400
Long-term debt	36,941	32,078
Carrying Amount [Member]
Financial Assets
Cash and due from banks	14,038	11,147
Federal funds sold and securities purchased under resale agreements	154	169
Investment securities held-to-maturity	42,030	43,590
Loans held for sale	2	74
Loans	264,715	256,986
Other financial instruments	2,339	2,311
Financial Liabilities
Deposits	317,590	300,400
Short-term borrowings	17,645	27,110
Long-term debt	36,941	32,078
Other Liabilities	1,408	1,353
Fair Value [Member]
Financial Assets
Cash and due from banks	14,038	11,147
Federal funds sold and securities purchased under resale agreements	154	169
Investment securities held-to-maturity	42,628	43,493
Loans held for sale	2	74
Loans	271,064	259,823
Other financial instruments	2,327	2,319
Financial Liabilities
Deposits	317,563	300,225
Short-term borrowings	17,470	26,782
Long-term debt	317,810	32,412
Other Liabilities	1,408	1,353
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	14,038	11,147
Investment securities held-to-maturity	3,398	2,275
Other financial instruments	0
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	154	169
Investment securities held-to-maturity	39,151	41,138
Other financial instruments	920	921
Financial Liabilities
Deposits	317,563	300,225
Short-term borrowings	17,470	26,782
Long-term debt	37,810	32,412
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	79	80
Loans held for sale	2	74
Loans	271,064	259,823
Other financial instruments	1,407	1,398
Financial Liabilities
Other Liabilities	$ 1,408	$ 1,353