Fair Values of Assets and Liabilities - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Cash and due from banks	$ 20,082	$ 19,505	$ 20,540	$ 15,705
Investment securities held-to-maturity	46,046	44,362
Loans	277,507	276,507
Financial Liabilities
Long-term debt	40,894	32,259
Carrying Amount [Member]
Financial Assets
Cash and due from banks	20,082	19,505
Federal funds sold and securities purchased under resale agreements	278	93
Investment securities held-to-maturity	46,046	44,362
Loans held for sale	1,305	20
Loans	277,507	276,507
Other	2,313	2,393
Financial Liabilities
Time deposits	38,935	33,356
Short-term borrowings	22,612	15,656
Long-term debt	40,894	32,259
Other	2,003	1,556
Fair Value [Member]
Financial Assets
Cash and due from banks	20,082	19,505
Federal funds sold and securities purchased under resale agreements	278	93
Investment securities held-to-maturity	44,332	43,723
Loans held for sale	1,305	20
Loans	280,398	279,391
Other	2,313	2,401
Financial Liabilities
Time deposits	38,605	33,120
Short-term borrowings	22,369	15,447
Long-term debt	40,259	32,377
Other	2,003	1,556
Fair Value [Member] | Level 1 [Member]
Financial Assets
Cash and due from banks	20,082	19,505
Investment securities held-to-maturity	4,505	4,613
Fair Value [Member] | Level 2 [Member]
Financial Assets
Federal funds sold and securities purchased under resale agreements	278	93
Investment securities held-to-maturity	39,815	39,095
Other	1,198	1,037
Financial Liabilities
Time deposits	38,605	33,120
Short-term borrowings	22,369	15,447
Long-term debt	40,259	32,377
Fair Value [Member] | Level 3 [Member]
Financial Assets
Investment securities held-to-maturity	12	15
Loans held for sale	1,305	20
Loans	280,398	279,391
Other	1,115	1,364
Financial Liabilities
Other	$ 2,003	$ 1,556