Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Financial Assets
Cash and due from banks	$ 20,319	$ 15,705	$ 10,981	$ 11,147
Investment securities held-to-maturity	43,393	42,991
Financial Liabilities
Deposits	336,873	334,590
Long-term debt	35,948	33,323
Carrying Amount [Member]
Financial Assets
Cash and due from banks	20,319	15,705
Federal funds sold and securities purchased under resale agreements	121	138
Investment securities held-to-maturity	43,393	42,991
Loans held for sale	51	4
Loans	269,761	269,394
Other financial instruments	2,311	2,362
Financial Liabilities
Deposits	336,873	334,590
Short-term borrowings	11,024	12,891
Long-term debt	35,948	33,323
Other Liabilities	1,586	1,702
Fair Value [Member]
Financial Assets
Cash and due from banks	20,319	15,705
Federal funds sold and securities purchased under resale agreements	121	138
Investment securities held-to-maturity	42,906	42,435
Loans held for sale	51	4
Loans	273,874	273,422
Other financial instruments	2,316	2,369
Financial Liabilities
Deposits	336,617	334,361
Short-term borrowings	10,843	12,706
Long-term debt	36,276	33,678
Other Liabilities	1,586	1,702
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	20,319	15,705
Investment securities held-to-maturity	4,974	4,605
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	121	138
Investment securities held-to-maturity	37,912	37,810
Other financial instruments	920	920
Financial Liabilities
Deposits	336,617	334,361
Short-term borrowings	10,843	12,706
Long-term debt	36,276	33,678
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	20	20
Loans held for sale	51	4
Loans	273,874	273,422
Other financial instruments	1,396	1,449
Financial Liabilities
Other Liabilities	$ 1,586	$ 1,702