Fair Value Measurements - Fair Value Disclosures of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
ASSETS
Other investments	$ 1,007	$ 1,064	$ 1,106
Loans, net of allowance	148	157	71
Derivative assets	475	693	771
LIABILITIES
Short-term borrowings	466	287	388
Derivative liabilities	450	584	657
Carrying Amount [Member]
ASSETS
Cash and short-term investments	4,283	4,525	3,182
Trading account assets	806	605	663
Securities available for sale	12,606	12,094	11,962
Held-to-maturity securities	4,835	3,931	4,153
Other investments	1,007	1,064	1,106
Loans, net of allowance	52,729	51,934	50,531
Loans held for sale	699	599	628
Mortgage servicing assets	331	204	188
Derivative assets	475	693	771
LIABILITIES
Deposits with no stated maturity	61,399	58,132	55,452
Time deposits	7,136	7,861	8,736
Short-term borrowings	1,921	1,896	2,134
Long-term debt	6,154	6,847	6,119
Derivative liabilities	450	584	657
Fair Value [Member]
ASSETS
Cash and short-term investments	4,283	4,525	3,182
Trading account assets	806	605	663
Securities available for sale	12,606	12,094	11,962
Held-to-maturity securities	4,730	3,992	4,212
Other investments	1,007	1,064	1,106
Loans, net of allowance	51,301	51,046	49,737
Loans held for sale	699	599	628
Mortgage servicing assets	388	238	237
Derivative assets	475	693	771
LIABILITIES
Deposits with no stated maturity	61,399	58,132	55,452
Time deposits	7,237	8,020	8,913
Short-term borrowings	1,921	1,896	2,134
Long-term debt	6,457	7,392	6,596
Derivative liabilities	450	584	657
Fair Value [Member] \| Level 1 [Member]
ASSETS
Cash and short-term investments	3,767	4,254	2,888
Trading account assets	6	2	10
Securities available for sale	24	43	27
Held-to-maturity securities
Other investments
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	60	54	52
LIABILITIES
Deposits with no stated maturity
Time deposits	605	408	569
Short-term borrowings	4		4
Long-term debt	6,192	2,807	2,819
Derivative liabilities	59	54	56
Fair Value [Member] \| Level 2 [Member]
ASSETS
Cash and short-term investments	516	271	294
Trading account assets	800	600	596
Securities available for sale	12,582	12,051	11,934
Held-to-maturity securities	4,730	3,992	4,212
Other investments	394	396	395
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	1,261	1,883	2,067
LIABILITIES
Deposits with no stated maturity	61,399	58,132	55,452
Time deposits	6,632	7,612	8,344
Short-term borrowings	1,917	1,896	2,130
Long-term debt	265	4,585	3,777
Derivative liabilities	948	1,331	1,504
Fair Value [Member] \| Level 3 [Member]
ASSETS
Cash and short-term investments
Trading account assets		3	57
Securities available for sale			1
Held-to-maturity securities
Other investments	613	668	711
Loans, net of allowance	51,301	51,046	49,737
Loans held for sale	699	599	628
Mortgage servicing assets	388	238	237
Derivative assets	23	26	28
LIABILITIES
Deposits with no stated maturity
Time deposits
Short-term borrowings
Long-term debt
Derivative liabilities	2	2	2
Fair Value [Member] \| Netting Adjustment [Member]
ASSETS
Cash and short-term investments
Trading account assets
Securities available for sale
Held-to-maturity securities
Other investments
Loans, net of allowance
Loans held for sale
Mortgage servicing assets
Derivative assets	(869)	(1,270)	(1,376)
LIABILITIES
Deposits with no stated maturity
Time deposits
Short-term borrowings
Long-term debt
Derivative liabilities	$ (559)	$ (803)	$ (905)

Deposits with no stated maturity fair value.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

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